Sometimes when importing accounts from another application, the SMTP (send) protocol is not there. If you go to Menu > Tools > Accounts, and click on the General tab for your account. You will see which protocols are available, and which are selected.
If SMTP is there but not selected, then just tick the box.
If SMTP is not there at all, you will need to setup the email account again by adding a new account. Once that is setup, you can delete the old one.