[exim] SMTP timeout sending mail to gmail

Nearly all (although not 100%) of my outgoing messages to gmail are giving
this (some are continuing to do this for 7 days now):

2006-04-06 18:48:45 1FRLeI-0006Wx-9T SMTP timeout while connected to
gmail-smtp-in.l.google.com [64.233.185.114] after end of data (1593 bytes
written): Connection timed out

How can I track this down? I'm using Exim 4.50 in Debian stable on Linux
2.6.15. I have a vague understanding of tcpdump, but not enough to give a
good trace to debug (let me know the right incantation and I'll send it
along).

I have no problems delivering to any other SMTP servers as far as I can tell.