From MozillaZine Knowledge Base

This article was written for Thunderbird but also applies to Mozilla Suite / SeaMonkey (though some menu sequences may differ).

An open mail relay is a 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 a 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 thier 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:

• If you're using your ISP as a 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. See Multiple SMTP servers for more information.

• "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. See the POP before SMTP support article for more information.

• "SMTP-AUTH" has the user provide a seperate username and password for the SMTP server. This may or may not be the same username/password used for the POP/IMAP server. If your email provider uses this method goto Tools -> Account Settings -> Outgoing Server (SMTP) and enter the username and then check the checkbox for "use name and password". See the SMTP authentication article for more information.

• You need to use a Virtual Private Network (VPN) to connect to the SMTP server. This requires you to configure Thunderbird to use the appropiate VPN client.

If you're unsure what method is used goto your email providers web site and browse thier 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. This was a very common problem before the GUI was improved in Thunderbird 1.5. You might want to use one of the extensions mentioned in the Multiple SMTP servers article to make it easier to switch SMTP servers or to tell which one you are using.

RFC 1893 defines a 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 thier 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.

See also

• Cannot send mail
• Mail utilities
• Multiple identities per account

External links

• SMTP relay behavior in Windows 2000, Windows XP, and Exchange Server
• Wikipedia article about open mail relays
• Exchange message restrictions