More detail required. What mail service, and what are the POP (receive) and SMTP (send) server settings? Port, SSL on or off, etc. Basically everything *except* your actual username/password.
And I assume you tried deleting and recreating the account in the Mail app? That'd be the first thing to try.
Note that POP isn't for sending, that a receive protocol. SMTP is the (typical) send protocol.
Brad.