[imp] getting error "unable to set sender to [emailaccount at domain]" after installing assp

[Prabal]

Hello,

I have a Windows 2003 server running Plesk 8.3, Mailenable with Horde IMP
webmail client. I have recently installed Anti-Spam SMTP Proxy (ASSP) (
http://www.asspsmtp.org/) on my server to counter SPAM. I have configured it
with the instructions found here -
http://www.hmailserver.com/forum/viewtopic.php?f=12&t=1881 . This
configuration works perfectly apart from one issue - whenever I try to send
emails from my Horde IMP webmail interface, I get an error saying "unable to
set sender to [emailaccount at domain]". If I deactivate ASSP everything works
fine.

I believe this is not an issue with ASSP since others don't have complaints.
Someone suggested it had something to do with how Horde IMP tries to send
emails.

Can someone provide any insight on what could be going wrong?

Thanks.