From MozillaZine Knowledge Base

How do I set up multiple SMTP (outgoing) mail servers?

Open the Edit menu and choose Mail & Newsgroup Account Settings. Choose Outgoing Server (SMTP). Click Advanced. Click Add to add new SMTP servers.

To set the outgoing server for an account, in the Mail & Newsgroup Account Settings dialog, choose Server Settings under the account. Click Advanced, choose a server, and click OK.

Official FAQ