Don't know if this will help, but I had an similar problem when trying to send email using a network different than my home ISP (and email account holder). (Note: I received mail on the secondary ISP with no problem.) Currently, when I'm connected to the secondary ISP and trying to email, I change my outgoing smtp to that of the secondary ISP server. There could have additional changes to my email account setup (as indicated by jortiz), but I don't recall. Sorry. Try googling for more info.

Good luck!