Upgrading from express to Outlook

I have about the same problem as Pam preceding this
question (I tried to reply to her message but got an error
msg). My problem was when I upgraded to Office XP and the
full outlook vs. my old express.

I remember the same option when installing (Corel settings)
and now I can receive e-mail in both Outlook and express
but can only send in express. My ISP says they only
support express and that my problem is with Microsoft.
They advised to check "My server requires authentication"
but I can't find that option in outlook.

Any Ideas?

Thanks,