perhaps they could also offer what they define as "Abusive Authentication Behavior" with that gem it will probably be simple. The forward is "probably" using a different SMTP server in the outgoing servers (SMTP) part of your account settings. So perhaps the issue is you are trying to send mail for one email address on the account for another. Thunderbird treats incoming account and outgoing accounts as two separate things and the identity in the incoming account (the pane above server settings in the account settings) is used to set the SMTP server for that incoming account identity. Frequently folk have those settings mixed and matched, especially if they maintain multiple identities for an account.