Though Thunderbird is an awesome email client though we get error sometimes of which 5.7.1 is most common.
This error indicates that the server doesn’t recognize you as a legitimate sender of email.
The most common cause is not being authenticated by the SMTP server. Some less likely possibilities are:
- You are blackballed (your domain is listed as a spammer by some anti-spam listing service).
- The recipient is a virtual user and there is some problem (on their POP/IMAP server) mapping the virtual address into a real address.
- The recipient might have some sort of mailbox restriction enabled such as only accepting messages from a distribution list, and the sender isn’t on that distribution list.
- Exchange 2003 has a feature to reject messages if they’re not from authenticated users.
- The DNS lookup failed.