We
had a similar issue. All of a sudden we could not download attachments
of more than approx. 600k. Ultimately, this was traced to a
fault in a new hub. I didn't put a sniffer on the network, so I can't
tell you what the hub was actually doing. I just know that before the
hub (Linksys 10/100 stackable) was added to our rack
everything worked, and after it was installed it didn't.
We had it in as a cheap temporary fix, and it caused more problems than it
solved. Replaced it with a nice HP and now everything is running
smooth...
Interestingly, this had the same issue with the Imail logs,
everything looked good, but the mail simply wasn't being downloaded or in
rarer cases delivered.
That probably doesn't help much but, you may want to look for general
networking issues rather than mail server issues if you are fairly
comfortable that the mail server is functioning
properly.
