Multiple SMTP servers - SeaMonkey
To set up multiple SMTP (outgoing) mail servers in Mozilla Suite/SeaMonkey mail, 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.