5.7.1 END-OF-MESSAGE: End-of-data rejected: user not permitted to relay. Please check the message and try again.
An open mail relay is an SMTP server configured to allow anyone on the internet to relay (send) a message using it. At one time this was a common configuration. However, nowadays that would be abused by spammers so many SMTP servers require the user to authenticate themselves whenever they send a message to somebody in another domain.
If you get an error message that looks roughly like "5.7.1 Unable to relay" or "550 5.7.1 Relaying prohibited" when trying to send a message it probably means the SMTP server couldn't authenticate you as a legitimate user. Most email providers don't care who uses their SMTP server to send a message to somebody in the same domain, so it may look like everything is working until you try to send a message to somebody in another domain.
Some common authentication methods:
- "SMTP-AUTH" has the user provide a separate username and password for the SMTP server. This is usually the same username/password used for the POP/IMAP server. Most email providers use this method. If your email provider uses this method goto Tools -> Account Settings -> Outgoing Server (SMTP), select the appropriate SMTP server, press the Edit button, and enter the username. Old versions of Thunderbird may also have a checkbox for "User name and password". If you have that checkbox, you need to check it.
- If you're using your ISP as an email provider it may check that you're logged into the internet using them. So this error might occur if you take your laptop somewhere else and use a public network. Setup another account with a free email provider such as Gmail and use that account's SMTP server with your existing account when traveling.
- "POP before SMTP" relies upon the user checking for new mail (thus logging in to the POP server) and then sending a message within X minutes.
- You need to use a Virtual Private Network (VPN) to connect to the SMTP server. This requires you to configure Postbox to use the appropriate VPN client.
If you're unsure what method is used goto your email providers web site and browse their email client support pages.
If you have multiple SMTP servers double check that you're actually using the SMTP server that you think you are.
RFC 1893 defines an enhanced set of error codes for delivery status notification. The vendor can provide whatever error message they want to supplement the error code, but 5.7.1 always means there is a permanent failure of "Delivery not authorized, message refused". The most common cause is not being authenticated by the SMTP server. Some less likely possibilities are:
- You're 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 has a feature to reject messages if they're not from authenticated users.
- The DNS lookup failed.