Cannot send e-mail thru outlook

My config is RH7.2 + Qmail+vpopmail +sqwebmail.
I can send & receive e-mail using my web browser.
When I try to use outlook as an e-mail client; I can only receive e-mail … I cannot send … why ???
I think I solved this problem before…. but I can’t remember..

Any help are welcomed.

ThanX in advance.