I am trying to consolidate emails using the Thunderbird identity feature. I have a single email account call it acct1@yahoo.com. I have three other email addresses that get forwarded to Yahoo, call them acct2@pobox.com, acct3@pobox.com, and acct4@hostmonster.com. I can send emails to all of these accounts. In Thunderbird yahoo is the default identity and the other three are alternate identities. The outgoing server settings are all identical (see screenshot). I can send emails using the acct1 and acct2 identities, but not the other two. This is puzzling because acct2 and acct3 are both forwarded identically. The error message I get is not very helpful: ''Sending of the message failed. An error occurred while sending mail. The mail server responded: Request failed; Mailbox unavailable. Please verify that your email address is correct in your account settings and try again.''