FermiMail FAQ: Troubleshooting
Email Headers Too Large
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Diagnostic-Code: smtp; 552 5.6.0 Headers too large (32768 max)"
Occasionally email messages are rejected by recipient systems due to message header size limitations
imposed by those recipient systems. This can happen when messages are routed through multiple
intermediate servers where each intermediate server adds its own header information. These headers
contain message forwarding data, system-specific diagnostic data, anti-spam or anti-virus data and
other various data. Cloud services like Exchange Online add quite a bit of header information, so
messages that flow between multiple cloud services can have larger than normal header sizes,
occasionally exceeding the arbitrary 32768-character limit.
Email RFC's 822, then 2822, then 5322 do not specify a maximum header size so the recipient's limit of
32768 characters is both common in many systems as a default setting, but is also completely arbitrary.
The recipients who failed to receive the message should request the administrators of their respective
email systems to increase the maximum header size of received messages to something larger than
32768, like 64K or whatever maximum size they have available in their system.
In FermiMail, messages that exit the Office 365 Exchange Online system, such as messages to the
Internet or messages sent to Listserv for relaying, have their “internal Exchange” headers removed in
order to reduce the header size by removing extraneous data that external systems can’t use.
Other than removing the internal Office 365 Exchange Online header data, there is nothing we can do to
limit the size of an email header as it passes through intermediate systems, growing along the way from
source to destination. If the message delivery path is long where a message passes through several
intermediate systems, the header can grow beyond the artificial limit imposed by some email systems.
If a message is rejected by an intermediate system or by a recipient system due to message header size,
the email administrators of those systems should refer to the RFC’s and increase the header size limit
they are imposing.