Mac Outlook Express 5.x Error 5550

This error can occur if Outlook Express is trying to send mail using an SMTP server that does not match the current ISP connection.  To resolve the problem...

1. Confirm which ISP is being used to establish the current Internet connection (dial-up or broadband).
2. Go to Tools > Accounts > [your account] > SMTP server and make sure this setting is compatible with the current Internet connection.
3. If this doesn't work, make sure that all message recipients are specified with valid, real e-mail addresses.