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MTU and Postfix


I've been busy on these days trying to solve a problem with Postfix that 
drove me nuts.

Sporadically (let's say one in hundred e-mails) my Postfix had problems 
for delivering messages with ~3 MiB of attachment to some e-mail hosts. 
DSN service returned the final notice of delivery to the user and logs 
displayed an error like "timed out while sending message body". 

These hosts were not of those "difficult" ones like Hotmail, Gmail, 
Yahoo! or the like that because to their high volume of traffic implement 
additional (and sometimes strambotic) measures to prevent spam and such 
"anti-all" systems that may require a different transport definiton in 
Postfix to get e-mails delivered. 

Moreover, these hosts were not e-mail servers that are behind Cisco PIX 
devices or using MS Exchange servers that are also well-known to be 
conflictive to "dialogue" with.

Nope, I was having problems for delivering to common, small hosts of mid-
size companies, one of the hosts running a Debian system, like mine. So I 
had to run some tests to find out what could be the problem here.

I first tried to define a less conservative values (by increasing the 
time) for "smtp_data_done_timeout", "smtp_data_xfer_timeout" and 
"smtp_data_init_timeout" but this had no effect at all and again, some e-
mails were still undelivered.

Googling around I found some posts and articles¹ pointing to the MTU 
value (my bonded interface was set by default to 1500) and as I had 
nothing to lose, I changed this and lowered to 1400.

This turned out to work wonders and since then (that's more than a week 
ago) I still had no other DSN delivery errors. Besides, e-mails in 
deferred queue that could not be sent in that time, after lowering the 
MTU value were also delivered with no apparent problems.

I'm still monitoring this but if this is the "cure" to prevent such 
errors, are there any expected drawbacks for lowering MTU "system-wide"?

The server has dual gigabit NIC which are bonded (in backup mode) and 
server itself is behind a FTTH gigabit router. The server also hosts a 
web server.

Any comments or experiences on this are welcome :-)




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