On Wed, Aug 31, 2011 at 1:09 PM, Camaleón <noela...@gmail.com> wrote:
> Hello, > > 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 :-) > > ¹http://www.hsc.fr/ressources/cours/postfix/doc/faq.html#timeouts > > You might want to try the postfix mailing list and see if they have any ideas. Be prepared for cold, hard, terse answers. They don't chat much - busy I guess. Was the previous MTU of 1500, a value you had set, or the default when queried? I'm wondering because of a recent experience I had tweaking MTU. I wanted to try jumbo frames to improve samba throughput on large video files. With MTU set to 9000 on Linux and Windows, throughput increased about 8 times. But it caused problems with the web service on Linux, which was running a domain under dyndns. I set the MTU on Linux back to unspecified, but left the jumbo frame active on Windows side. The performance was still very good in large samba file transfers. I might remember this wrong, but it seemed it was worse performance when Linux side specified 1500, so I left it as unspecified and it has worked well. I still get high transfer speeds in samba with unspecified MTU on Linux but jumbo of 9000 MTU on Windows side. BTW, 1492 is a common MTU seen in FAQs. You might get just as good with that.
