Mostly for the archives, but also for anyone else out there who
is having this problem and might like a solution:

I am running NetBSD-1.5.1/i386 and use qmail.  Some time recently I
stopped being able to relay mail via BigPond's mail server.

netstat showed that connections were made to port 25, but that
outgoing queued data wasn't being sent.  I could manually send mail
using telnet to port 25 which suggested it was a TCP problem and not a
mail configuration problem, which seemed pretty odd.

Lazily I didn't take a network trace to find out exactly what was
happening, but adjusting the following sysctl values made things work:

net.inet.ip.mtudisc=1
net.inet.tcp.mss_ifmtu=1

I don't claim that this was scientific troubleshooting ... :-)

Regards,

Giles