SMTP enhanced status code 5.7.23

What the registry says

The basic codes it travels with

• 550 Requested action not taken: mailbox unavailable (e.g., mailbox not found, no access, or command rejected for policy reasons)

In practice

X.7.23 names an SPF fail directly. Microsoft uses 5.7.23 when the destination uses SPF and your configuration fails, and Gmail uses 451 4.7.23 for a missing PTR record. For SPF, check that every service sending as your domain is included, and that the record stays within the 10-lookup limit.

What the sender should do

Add the sending service's IPs or include mechanism to your SPF record, and make sure the record has no syntax errors.

How providers use X.7.23

Quoted from each provider’s own error reference.

• Gmail 451 4.7.23

  ip-address The sending IP address for this message doesn't have a PTR record, or the PTR record's forward DNS entry doesn't match the sending IP address. To protect users from spam, your email has been temporarily rate limited. To learn more about IP address requirements for sending to Gmail, visit the IP addresses section of our sender guidelines. To learn more about Gmail requirements for bulk email senders, visit Email sender guidelines.

• Microsoft 365 5.7.23

  The message was rejected because of Sender Policy Framework violation

Common questions

What does 5.7.23 mean?

X.7.23 means the message failed an SPF check, and the receiver's policy rejects SPF failures. It names SPF specifically where older servers used 5.7.1.

What should I do about 5.7.23?

Add the sending service's IPs or include mechanism to your SPF record, and make sure the record has no syntax errors.

Would an email verifier have caught 5.7.23?

No. This reply is about your sending setup, the message or the receiving system, not about whether the address exists, so verifying the list will not prevent it.

Full guides for this code

• 550 5.7.23 — SPF violation (Microsoft 365)