Problem
Many email systems use a rudimentary loop-prevention technique based on a simple count
of Received: header entries. Any message exceeding a threshold value is rejected, on the assumption it
is looping.
As email services add layers, older threshold settings become
inadequate. Because it passes through multiple layers both inbound and outbound, mail
sent via IEEE lists is disrupted more easily than direct exchanges. IEEE list
email carries a minimum of eight entries, and commonly up to 14, at the first hop beyond
the IEEE domain. Some is rejected because of hop count before it reaches the subscriber.
RFC 5321 (Simple Mail Transfer Protocol, the draft standard specification for email)
anticipates this; subclause 6.3 states: SMTP servers using this technique SHOULD use a
large rejection threshold, normally at least 100 Received entries. The same statement was in the
previous version, RFC 2821, so this recommendation has been stable for nearly a decade. Yet
many systems use default threshold settings around 16.
Recommendations
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For IEEE 802.1 list subscribers, thresholds should allow a message leaving the IEEEs
domain carrying 16 Received: entries to reach the subscriber. That number is minimally
safe now, but may not remain so for long. In the long run, allowing for 20
or more would offer a better chance of avoiding more problems.
Note this may require considerably higher thresholds for deeply nested or forwarded addresses.
Note also that in deeply nested configurations, adjusting the threshold only in one server
may prove ineffective. Mail may be rejected before it reaches that server, or it may be rejected further
downstream if the change allows it to reach other servers with inadequate threshold settings after accumulating
more hops. Adjustments are needed throughout the email domain.
Email administrators may resist the change because they have statistics showing a significant rate of
looping email being detected and discarded. In this case, they need to take into account that some of this
discarded email is known to be non-looping and discarded in error.
In some cases, the problem may be caused or made worse by lengthy email-forwarding arrangements.
Subscribing a more direct address may fix it, or at least reduce the rejection rate.
Contact for more information
The 802.1 list administrator is available for deeper discussion of this problem
and correction strategies for specific cases.
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