[imp] Can a firewall cause a timeout for sending mail?

[Otto Stolz]

Hello,

one of my customers cannot send mail via IMP 3.2.2
from a particular computer, whilst reading and deleting
messages poses no problem. (And, of course, he can send
mail from another computer.)

After waiting for 2 minutes, he'll receive a popup
message from Imp saying: "Das Dokument enthält keine Daten",
or so he says -- however, this string is neither in
horde/imp/po/de_DE.po nor in horde/po/de_DE.po
(it translates to "This document contains no data").

In the horde error log, I find:
> Oct 11 17:21:20 2006 [client xx.xx.xx.xx]
>                      read request line timed out
Now, I conjecture that some firewall is blocking the
upload of the message text.

Questions:
- Which TCP port (or other mechanism) is used to upload
   the message body? Anything different from the usual
   HTTPS port?
- How should a firewall be configured to allow for sending
   mail via Imp 3.2?
- Will this be any different with Imp 4.1?

Thank you for your time and consideration,
   Otto Stolz