What does "550 <>: Sender address rejected: Please see..." mean?

Nearly all spam, scam & virus e-mail messages carry fake sender addresses. The users whose "From" addresses are being abused, often suffer from the consequences, wasting their time sorting out misdirected bounce messages or replies.

SPF is an extension to Internet e-mail, which aims to prevent unauthorized people from forging the e-mail "From" address. For more detailed information, see Sender Policy Framework Introduction.

When you see a message like the following example, it indicates that your email is coming from an IP address which is not registered (using the SPF protocol), to the domain you're sending "From".

Example

550 5.7.1 <sender@example.com>: Sender address rejected: Please see http://www.openspf.org/Why?id=sender%40example.com&ip=1.2.3.4&receiver=an.mta.example.com


Explanation

an.mta.example.com rejected a message that claimed an envelope sender address of sender@example.com.

an.mta.example.com received a message from 1.2.3.4 that claimed an envelope sender address of sender@example.com.

However, the domain example.com has declared using SPF that it does not send mail through 1.2.3.4. That is why the message was rejected.

This particularly affects mobile and home workers, who must ensure their email is sent via their corporate or ISP mail gateway and not directly. These users should contact their network administrator for correct settings.

Also, people with domains registered to a 3rd-party forwarding service, that then re-address the email and send it on, may experience problems if the 3rd-party's IP address(es) are not registered in the sender domain's SPF record. Receivers can arrange their forwarding in a way that works with SPF, by rewriting the "MAIL FROM" at the forwarder (as done by mailing lists). The Sender Rewriting Scheme is one way to achieve this.