[imp] IMP webmail problem

[Billy Dimaculangan]

we're using IMP(horde) for remote mail access, but whenever we send mail
from this webmail the from field appears to myuser at localhost.mydomain.com.
the problem is replying to this "myuser at localhost.mydomain.com" would cause
the mail to bouce back stating that "Domain of sender address
myuser at localhost.mydomain.com does not exist".
what am i missing?

--
Billy