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 -- ## List details at http://www.exim.org/mailman/listinfo/exim-users ## Exim details at http://www.exim.org/ ## Please use the Wiki with this list - http://www.exim.org/eximwiki/
