[imp] Qmail, Plesk, IMP: can't send mail

[Rich Churcher]

I've done a fairly thorough search of the archives, and web, and newsgroups,
and various web forums without success.  I see a few items with similar
symptoms to my problem, but nothing quite fits the bill.  So here's the
thing.

RH 7.3 box with Plesk PSA 5.
php 4.1.2-7.3.6 (RPM)
psa-qmail-1.03-rh7.3.build030207.16 (RPM)
apache-1.3.27-2 (RPM)
IMP 3.0
Horde 2.0

I wish I could dispense with PSA altogether, but people seem to insist on
control panels these days.

IMP works well for receiving, but refuses to send mail.  The composition
window, once send is clicked, refreshes to a blank email message instead of
closing.  No mail is ever sent, or at least none ends up at the destination.

Maillog looks rather like this:

Aug 11 08:29:01 ithaca imapd: Connection, ip=[127.0.0.1]
Aug 11 08:29:01 ithaca imapd: LOGIN, user=rich, ip=[127.0.0.1]
Aug 11 08:29:01 ithaca imapd: LOGOUT, user=rich, ip=[127.0.0.1],
 headers=0, body=0
Aug 11 08:29:21 ithaca imapd: Connection, ip=[127.0.0.1]
Aug 11 08:29:21 ithaca imapd: LOGIN, user=rich, ip=[127.0.0.1]
Aug 11 08:29:21 ithaca imapd: LOGOUT, user=rich, ip=[127.0.0.1],
 headers=0, body=0

httpd logs contain nothing remarkable.  There is nothing in the qmail queue.
The mail for all domains has been tested via ordinary mail client and works
well for both sending and receiving.

I'd love to get away without upgrading PHP for a few more weeks if possible,
until I complete a few other tasks.  Is there a simple fix I'm missing for
this issue?

--
Cheers,
Rich.