Re: [exim] SMTP data timeout

Chris Perry wrote:
> Thanks for elaborating. I suspect the MTU is not the issue at this
> time. I did have problems with the MTU when I started with DLS, and got
> that sorted, with assistance from the ISP. Today I was able to send a
> large email to the affected exim server without issue. The problem
> seems to centre on some relationship between the sending (UBS) and
> receiving servers.

It could still be an MTU issue at their end, or a Path MTU Discovery
issue somewhere on the route between you and them. The symptoms you've
described definitely sound like MTU issues. Can you disable Path MTU
Discovery in your TCP stack? On FreeBSD you would do this by setting
the net.inet.tcp.path_mtu_discovery sysctl variable to 0. I've found
that this is a good way to find out if PMTUD is the problem or not - if
the problem messages come through successfully with PMTUD disabled, you
can further troubleshoot. It seems, though, that the problem may
actually be at the sending end of things, so you may need to try to get
them to disable PMTUD on their sending server and see if it works
better, which may be difficult to arrange.

Regards,
Tim Wilde