[imp] Sending message error in IMP

[Adrian Mak]

I composed a message and clicked send, IMP showed and error
"There was an error sending your message: sendmail [/usr/lib/sendmail]
is not a valid file"

My system is redhat AS3 and my mail system is running
postfix 2.2.5 + Dovecot + cyrus-sasl + pam-mysql + mysql + virtual mailbox

All email accounts are stored in mysql db and email users are
authenticated against mysql db thur pam-mysql