I was setting up Thunderbird at home with wired Internet connection, and everything was fine.
However when I tried getting new mails using college wireless which requires proxy with authentication, after typed my username and password, Thunderbird status bar showed some progress, then I received this error:
Could not connect to mail server email@example.com, the connection was refused.
Ironically, I still be able to open tab for browsing some addons, but could not able to download them. I assumed since Firefox works so well under proxy (to both Thunderbird and FF, I used the setting in Edit -> Pref -> Advance -> Network Tab -> Connection), Thunderbird would conduct the same and work fine.
I'm currently using Ubuntu Oneiric.