Mail Services
Mail Services

Three major mail services:

Simple Mail Transfer Protocol


Post Office Protocol


SMTP
POP3
Internet Mail Access Protocol

IMAP or IMAP4
Which one to use?

Outbound Mail

SMTP for outbound email


Port 25 or 2525
Inbound Mail

POP3 for inbound email


Port 110
IMAP for inbound email

Port 143
What is SMTP?

Simple Mail Transfer Protocol (SMTP) is the
standard protocol for sending emails across the
Internet.


SMTP uses TCP port 25 or 2525
Sometimes you may have problems sending messages


ISP may have closed port 25
To determine the SMTP server for a given domain name

MX (Mail eXchange) DNS record is used
IMAP or POP3?


Both receive email
Main differences between IMAP and POP3:


POP3 protocol assumes there is only one client
connected to the mailbox
IMAP protocol allows simultaneous access by
multiple clients

IMAP is suitable if the mailbox is used by multiple
users
SMTP
Material from:
http://en.wikipedia.org/wiki/Smtp
SMTP

Simple Mail Transfer Protocol


The de facto standard for e-mail transmissions across the
Internet.
Defined in RFC 821 (STD 10)


amended by RFC 1123 (STD 3) chapter 5.
The protocol used today is also known as ESMTP

Defined in RFC 2821.
SMTP

Relatively simple text-based protocol

One or more recipients of a message are specified



In most cases verified to exist
The message text is transferred
Client-server protocol


The client transmits an email message to the server
Either



an end-user's email client, a.k.a. MUA (Mail User Agent),
- or a relaying server's MTA (Mail Transfer Agents)
can act as an SMTP client.
SMTP


An email client knows the outgoing mail SMTP server from
its configuration
A relaying server typically determines which SMTP server
to connect to by looking up the MX (Mail eXchange) DNS
record for each recipient's domain name


Conformant MTAs (not all) fall back to a simple A record
in the case of no MX


A Record: address record
Some current mail transfer agents will also use SRV records




The part of the email address to the right of the at (@) sign
SRV Records: SeRVice records
More general form of MX,
These are not widely adopted.
Relaying servers can also be configured to use a smart host
SMTP

The SMTP client initiates a TCP connection
to the server's port 25


unless overridden by configuration
It is quite easy to test an SMTP server using
the telnet program

see following example
SMTP

SMTP is a "push" protocol that does not allow
one to "pull" messages from a remote server
on demand



That is it sends only
To do a pull (i.e. receive) a mail client must
use POP3 or IMAP
Another SMTP server can trigger a delivery in
SMTP using ETRN
Outgoing mail SMTP server



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
An email client requires the name or the IP address of an
SMTP server as part of its configuration
Server will deliver messages on behalf of the user
Setting allows for various policies and network designs
End users connected to the Internet can use the services of an
e-mail provider that is not necessarily the same as their
connection provider
Network topology, or the location of a client within a network
or outside of a network, is no longer a limiting factor for
email submission or delivery
Modern SMTP servers typically use a client's credentials
(authentication) rather than a client's location (IP address), to
determine whether it is eligible to relay email.
Outgoing mail SMTP server

Another choice is whether to use TCP port 25 (SMTP) or
port 587 (Submission), as established by RFC 2476, for
relaying outbound mail to a mail server.


Some servers still support port 465 for legacy secure SMTP


It is preferable to use encryption on standard ports according to RFC
2487.
Some servers are setup to reject all relaying on port 25


Many servers support both.
Valid users authenticating on port 587 are allowed to relay mail to
any valid address.
A server that relays all email for all destinations for all clients
connecting to port 25 is known as an open relay

Generally considered a bad practice worthy of blacklisting.
Sample communications

After establishing a connection between the sender
(the client) and the receiver (the server), the
following page shows a legal SMTP session.


In the following conversation, everything sent by the
client is prefaced with C: and everything sent by the
server is prefaced with S:.
On most computer systems, a connection can be
established using the telnet command on the client
machine, for example:


telnet www.example.com 25
Opens a TCP connection from the sending machine to the MTA
listening on port 25 on host www.example.com.
Sample communications
S:
C:
S:
C:
S:
C:
S:
C:
S:
C:
C:
C:
C:
C:
C:
C:
C:
S:
C:
S:
220 www.example.com ESMTP Postfix
HELO mydomain.com
250 Hello mydomain.com
MAIL FROM:<sender@mydomain.com>
250 Ok
RCPT TO:<friend@example.com>
250 Ok
DATA
354 End data with <CR><LF>.<CR><LF>
Subject: test message
From: sender@mydomain.com
To: friend@example.com
Hello,
This is a test.
Goodbye.
.
250 Ok: queued as 12345
QUIT
221 Bye
Sample communications

Note: the data the client sends in the HELO and
MAIL FROM commands can be retrieved in
additional headers that the server adds to the
message:


Received and Return-Path respectively.
Optional: (and not shown above) nearly all clients
ask the server which SMTP extensions the server
supports


Uses the EHLO greeting to invoke Extended SMTP
(ESMTP).
These clients use HELO only if the server does not
respond to EHLO.
Sample communications


Contemporary clients will use the ESMTP
extension keyword SIZE to inquire of the
server the maximum message size that will be
accepted.
Older clients and servers

Try to transfer huge messages that will be
rejected


After wasting the network resources
Including a lot of connect time to dialup ISPs

paid by the minute.
Sample communications

For the edit planning of giant files or sending with older clients, users can
manually determine in advance the maximum size accepted by ESMTP
servers.

The user telnets as above, but substitutes "EHLO mydomain.com" for the
HELO command line:





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S: 220-serverdomain.com ESMTP {postfix version and
date}
S: 220 NO UCE. {etc., terms of service}
C: EHLO mydomain.com
S: 250-serverdomain.com Hello mydomain.com [127.0.0.1]
S: 250-SIZE 14680064
S: 250-PIPELINING
S: 250 HELP
This serverdomain.com declares that it will accept a fixed maximum
message size no larger than 14,680,064 octets (8-bit bytes).
Depending on the server's actual resource usage, it may be currently
unable to accept a message this large.
Sample communications


In the simplest case, an ESMTP server will declare a
maximum SIZE with only the EHLO user
interaction.
If no number appears after the SIZE keyword, or if
the current message limit must be exactly
determined:


User can further interact by simulating the ESMTP header
of a message with an estimated size.
See External Link RFC 1870
POP
From:
http://en.wikipedia.org/wiki/Post_Office_Protocol
POP

Local e-mail clients use the Post Office Protocol
version 3 (POP3)



An application-layer Internet standard protocol
Retrieves e-mail from a remote server over a TCP/IP
connection
Many subscribers to individual Internet service
provider e-mail accounts access their e-mail with
client software that uses POP3.
POP Overview

POP3 has made earlier versions of the
protocol obsolete


POP (informally called POP1 and POP2)
Now, the term POP almost always means POP3
in the context of e-mail protocols
POP Overview

The design of POP3 and its procedures supports endusers with intermittent connections (such as dial-up
connections)



Allows users to retrieve e-mail when connected
View and manipulate the retrieved messages without
needing to stay connected
Although most clients have an option to leave mail
on server, e-mail clients using POP3 generally:





Connect
Retrieve all messages
Store them on the user's PC as new messages
Delete them from the server
Disconnect.
POP Overview

In contrast, the newer, more capable Internet
Message Access Protocol (IMAP) supports both
connected and disconnected modes of operation.


E-mail clients using IMAP generally leave messages on
the server until the user explicitly deletes them.
This and other facets of IMAP operation allow
multiple clients to access the same mailbox
POP Overview

Most e-mail clients support either POP3 or IMAP to retrieve
messages


Few Internet Service Providers (ISPs) support IMAP
The fundamental difference between POP3 and IMAP4:

POP3 offers access to a mail drop




Mail exists on the server until it is collected by the client
If the client leaves some or all messages on the server
The client's message store is considered authoritative
In contrast, IMAP4 offers access to the mail store



The client may store local copies of the messages
These are considered to be a temporary cache
The server's store is authoritative
POP Overview

Clients with a leave mail on server option generally use the POP3 UIDL (Unique
IDentification Listing) command.


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Most POP3 commands identify specific messages by their ordinal number on the mail server.
Creates a problem for a client intending to leave messages on the server
Message numbers may change from one connection to the server to another
For example if a mailbox contains five messages at last connect, and a different client then
deletes message #3, the next connecting user will find the last two messages' numbers
decremented by one
UIDL provides a mechanism to avoid these numbering issues.



Server assigns a string of characters as a permanent and unique ID for the message
When a POP3-compatible e-mail client connects to the server, it can use the UIDL command
to get the current mapping from these message IDs to the ordinal message numbers
The client can then use this mapping to determine which messages it has yet to download


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The advantage of the numeric UID is with large mailboxes


Saves time when downloading
Note: IMAP has a similar mechanism, using a 32-bit UID (Unique IDentifier) that is required to be strictly ascending.
A client can request just the UIDs greater than its previously stored "highest UID"
In POP, the client must fetch the entire UIDL map
POP Overview

Whether using POP3 or IMAP to retrieve messages:

E-mail clients typically use the SMTP_Submit profile of the SMTP protocol to send
messages

E-mail clients are commonly categorized as either POP or IMAP clients, but
in both cases the clients also use SMTP
There are extensions to POP3 that allow some clients to transmit outbound
mail via POP3



Known as "XTND XMIT" extensions.

The Qualcomm qpopper and CommuniGate Pro servers and Eudora clients are
examples of systems that optionally utilize the XTND XMIT methods of
authenticated client-to-server e-mail transmission.
MIME serves as the standard for attachments and non-ASCII text in email

Although neither POP3 nor SMTP require MIME-formatted e-mail,
essentially all Internet e-mail comes MIME-formatted


POP clients must also understand and use MIME
IMAP, by design, assumes MIME-formatted e-mail
POP Overview

POP3 originally supported only an unencrypted login mechanism

Although plain text transmission of passwords in POP3 still commonly
occurs, POP3 currently supports several authentication methods to provide
varying levels of protection against illegitimate access to a user's e-mail




One such method, APOP, uses the MD5 hash function in an attempt to avoid
replay attacks and disclosure of the shared secret
Clients implementing APOP include Mozilla Thunderbird, Opera, Eudora, KMail
and Novell Evolution
POP3 clients can also support SASL authentication methods via the AUTH
extension. MIT Project Athena also produced a Kerberized version
POP3 works over a TCP/IP connection using TCP on network port 110



E-mail clients can encrypt POP3 traffic using TLS or SSL
A TLS or SSL connection is negotiated using the STLS command
Some clients and servers, like Google Gmail, instead use the deprecated
alternate-port method, which uses TCP port 995
POP Example (APOP)
S: <wait for connection on TCP port 110>
C: <open connection>
S: +OK POP3 server ready <1896.697170952@dbc.mtview.ca.us>
C: APOP mrose c4c9334bac560ecc979e58001b3e22fb
S: +OK mrose's maildrop has 2 messages (320 octets)
C: STAT
S: +OK 2 320
C: LIST S: +OK 2 messages (320 octets)
S: 1 120
S: 2 200
S: .
C: RETR 1
S: +OK 120 octets
S: <the POP3 server sends message 1>
S: . C: DELE 1
S: +OK message 1 deleted
C: RETR 2
S: +OK 200 octets
S: <the POP3 server sends message 2>
S: . C: DELE 2
S: +OK message 2 deleted
C: QUIT S: +OK dewey POP3 server signing off (maildrop empty)
C: <close connection> S: <wait for next connection>
POP Example

POP3 servers without the optional APOP
command expect you to log in with the
USER and PASS commands:
C: USER mrose
S: +OK User accepted
C: PASS mrosepass
S: +OK Pass accepted
IMAP
IMAP

Internet Message Access Protocol

Commonly known as IMAP or IMAP4

An application layer Internet protocol





Previously called Internet Mail Access Protocol, Interactive Mail
Access Protocol (RFC 1064), and Interim Mail Access Protocol)
Current version is IMAP version 4 revision 1


Operates on port 143
Allows a local client to access e-mail on a remote server
IMAP4rev1: defined by RFC 3501
IMAP4 and POP3 are the two most prevalent Internet
standard protocols for e-mail retrieval
Virtually all modern e-mail clients and servers support both
IMAP


IMAP supports both connected and disconnected modes of
operation.
E-mail clients using IMAP generally leave messages on the
server until the user explicitly deletes them.


Most e-mail clients support either POP3 or IMAP


This and other facets of IMAP operation allow multiple clients to
access the same mailbox.
Fewer Internet Service Providers (ISPs) support IMAP.
IMAP4 offers access to the mail store



the client may store local copies of the messages
considered to be a temporary cache
the server's store is authoritative.
IMAP

E-mail messages are generally sent to an e-mail server



User later retrieves these messages with either a web browser or an e-mail
client that uses one of a number of e-mail retrieval protocols.
Some clients and servers preferentially use vendor specific, typically
proprietary protocols, most support the Internet standard protocols




SMTP for sending e-mail
POP3 and IMAP4 for retrieving e-mail
Allows interoperability with other servers and clients
Proprietary protocols:




Stores received messages in the recipient's e-mail mailbox
Microsoft Outlook client when communicating with an Exchange server
IBM Notes client when communicating with a Domino server
Both products also support SMTP, POP3, and IMAP4.
Support for the Internet standard protocols



Allows other e-mail clients to access these servers
Allows the clients to be used with other servers
E.g. Qualcomm's Eudora or Mozilla Thunderbird
IMAP

E-mail clients can generally be configured to use either POP3 or IMAP4
to retrieve e-mail and in both cases use SMTP for sending.



Many (if not most) subscribers to individual ISP e-mail accounts access
their e-mail with client software that uses POP3
IMAP is often used in large networks


For example, a college campus mail system.
IMAP allows users to access new messages instantly on their computers


Most e-mail programs can also use Lightweight Directory Access Protocol
(LDAP) for directory services
The mail is stored on the network
With POP3, users either




download the e-mail to their computer
access it via the web.
Both methods take longer than IMAP
User must either download any new mail or "refresh" the page to see the new
messages.
Advantages over POP3

Connected and disconnected modes of operation




When using POP3, clients typically connect to the e-mail server
briefly, only as long as it takes to download new messages.
When using IMAP4, clients often stay connected as long as the user
interface is active and download message content on demand.
For users with many or large messages, this IMAP4 usage pattern can
result in faster response times.
Multiple clients simultaneously connected to the same
mailbox


The POP3 protocol requires the currently connected client to be the
only client connected to the mailbox.
The IMAP protocol specifically allows simultaneous access by
multiple clients

Provides mechanisms for clients to detect changes made to the mailbox
by other, concurrently connected, clients
Advantages over POP3

Access to MIME message parts and partial fetch



Nearly all internet e-mail is transmitted in MIME format,
allowing messages to have a tree structure where the leaf
nodes are any of a variety of single part content types and
the non-leaf nodes are any of a variety of multipart types.
The IMAP4 protocol allows clients to separately retrieve
any of the individual MIME parts and also to retrieve
portions of either individual parts or the entire message.
These mechanisms allow clients to retrieve the text
portion of a message without retrieving attached files or to
stream content as it is being fetched.
Advantages over POP3

Message state information

flags (defined in the IMAP4 protocol clients) can keep track of message state


Flags are stored on the server



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If a single user accesses a mailbox with two different POP3 clients
State information, such as whether a message has been accessed, cannot be synchronized
between the clients
The IMAP4 protocol supports both pre-defined system flags and client defined
keywords


Different clients accessing the same mailbox at different times can detect state changes made
by other clients
POP3 provides no mechanism for clients to store such state information on the server


for example whether or not the message has been read, replied to, or deleted
System flags indicate state information such as whether a message has been read.
Keywords, (not supported by all IMAP servers), allow messages to be given one or
more tags whose meaning is up to the client
Adding user created tags to messages is an operation supported by some Webmail
services, such as Gmail

Generally not using IMAP.
Advantages over POP3

Multiple mailboxes on the server



Server-side searches



IMAP4 clients can create, rename, and/or delete mailboxes (usually
presented to the user as folders) on the server, and move messages between
mailboxes.
Multiple mailbox support also allows servers to provide access to shared
and public folders.
IMAP4 provides a mechanism for a client to ask the server to search for
messages meeting a variety of criteria
Avoids requiring clients to download every message in the mailbox in
order to perform searches
Built-in extension mechanism


IMAP4 defines an explicit mechanism by which it may be extended.
Many extensions to the base protocol have been proposed and are in
common use.


IMAP2bis did not have an extension mechanism
POP3 now has one defined by RFC 2449.
Disadvantages of IMAP

IMAP remedies many of the shortcomings of
POP


Inherently introduces additional complexity
Much of this complexity (e.g., multiple clients
accessing the same mailbox at the same time)
is compensated for by server-side
workarounds


Maildir
Database backends
Disadvantages of IMAP

Unless the mail store and searching
algorithms on the server are carefully
implemented, a client can potentially consume
large amounts of server resources when
searching massive mailboxes.
Disadvantages of IMAP

IMAP4 clients need to explicitly request new email
message content



Potentially causing additional delays on slow connections
such as those commonly used by mobile devices.
A private proposal, push IMAP, would extend IMAP
to implement push e-mail by sending the entire
message instead of just a notification.
However, push IMAP has not been generally
accepted

Current IETF work has addressed the problem in other
ways
Disadvantages of IMAP

Unlike some proprietary protocols which combine sending
and retrieval operations, sending a message and saving a copy
in a server-side folder with a base-level IMAP client requires
transmitting the message content twice, once to SMTP for
delivery and a second time to IMAP to store in a sent mail
folder.




Remedied by a set of extensions defined by the IETF LEMONADE
Working Group for mobile devices
POP3 servers don't support server-side folders so clients have no
choice but to store sent items on the client.
Many IMAP clients can be configured to store sent mail in a clientside folder.
Courier Mail Server offers a non-standard method of sending using
IMAP by copying an outgoing message to a dedicated outbox folder.
SMTP-AUTH
SMTP Authentication
SMTP Authentication

SMTP-AUTH

Extension of the Simple Mail Transfer Protocol (SMTP)




Includes an authentication step
Client effectively logs in to the mail server during the
process of sending mail
Servers which support SMTP-AUTH can usually be
configured to require clients to use this extension,
ensuring the true identity of the sender is known.
SMTP-AUTH is defined in RFC 2554.
SMTP Authentication

SMTP-AUTH provides an access control
mechanism


Can be used to allow legitimate users to relay
mail while denying relay service to unauthorized
users, such as spammers
Does not guarantee the authenticity of either the
SMTP envelope sender or the "From:" header.

Spoofing, in which one sender masquerades as
someone else, is possible even with SMTP-AUTH.
SMTP Authentication

SMTP-AUTH extension also allows one mail
server to indicate to another that the sender has
been authenticated when relaying mail

This requires the recipient server to trust the
sending server


This aspect of SMTP-AUTH is rarely used on the
Internet.
The recipient of an e-mail message cannot tell
whether the sender was authenticated

Use of SMTP-AUTH is only a partial solution to spam.
SMTP Authentication

While SMTP-AUTH is generally a security
improvement over unauthenticated SMTP, it can
also introduce a weakness

If authenticated users are allowed to submit messages
from IP addresses where unauthenticated users are not —
that is, if authenticated users are allowed to relay mail —
then an attacker who subverts one user's account is then
able to use the authenticated server as an open mail relay.



In such a configuration every user's password becomes a key to
the mail system's security.
Spammers have attacked SMTP-AUTH mail servers by
bruteforcing common usernames and passwords
A good password policy can effectively prevent such an
attack
Acronyms
Acronym summary



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
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
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ESMTP - Extended SMTP
ETRN - Extended Turn
IMAP - Internet Mail Access Protocol
MTA - Mail Transfer Agents
MUA - Mail User Agent
MX
- Mail eXchange
POP
- Post Office Protocol
SMTP - Simple Mail Transfer Protocol
UIDL - Unique IDentification Listing