What is Thunderbird Email error 5.7.1

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.