Re: [exim] SMTP timeout sending mail to gmail

On Sun, 9 Apr 2006, Daniel Webb wrote:

> Here's what I think is happening, and why it was only manifesting
> with Google: Google is filtering ICMP packets used for path MTU
> discovery and thus breaking emails that go over 400 bytes (only with
> my system). Because the mail server had an MTU of 1500, Google was
> trying to use 1500 byte packets.

We used to encounter very similar symptoms with a very small number of
remote sites. In our case, it wasn't unusual for small mails to get
through, while large ones were retried indefinitely and finally
timed-out.

On investigation it usually transpired that the destination site was
using a Cisco PIX firewall, with what I suppose was a default (but in
any case inept!) configuration which, as you say, defeated MTU
discovery. Temporarily reducing the MTU size in our networking stack
would get the items through, but we refused to nobble our whole
networking performance just for the sake of one or two misbehaved
peers. I played around for a while trying to configure explicit
routes to the rogue destinations and give them an explicit MTU, but
never came up with an answer that I could really recommend to anyone
else.

The problem seemed to fade away with time, so I stopped looking for
Wuergarounds (to use a rather apt piece of German techno-slang).

Seems you may have encountered it in a different context...

OT here, but surely it must be possible to control your networking
software's use of RTP (UDP) packet size as desired for VoIP,
separately from the values used by TCP?

good luck