smtp;550 5.1.1 Not our Customer
The server will not relay mail for this destination. Send through the correct outbound server for the domain, and confirm the recipient really hosts mail with this provider.
Provider documentation
Email provider · 5.x.x permanent failure
Comcast SMTP 550 Errors: Every Bounce Response Explained
Every SMTP 550 response on record from Comcast (comcast.net), with the verbatim text, the enhanced status code, a severity rating and a plain-English fix.
The mailbox is unavailable and the message is rejected for good. This is the catch-all permanent failure, covering everything from an unknown address to an authentication failure to a reputation block, so the wording after the number is what actually tells you the reason.
- Provider
- Comcast
- Code
- 550 (5xx)
- Responses
- 4
- Highest severity
- High
Comcast SMTP 550 responses
Status 5.1.1-2#
smtp;550 5.1.1 <[email protected]> recipient mailbox unallocated
The address does not exist, or is no longer active, at this provider. Remove it from your list and check for a typo in the local part or the domain.
Provider documentation
smtp;550 5.2.0 Message rejected due to DMARC. Please see http://postmaster.comcast.net/smtp-error-codes.php#DM000001
The message did not pass the sending domain's DMARC policy, so the receiver rejected it. Sign the mail with DKIM for your From domain, or align SPF through a custom Return-Path, then send again.
Status 5.2.0-2#
smtp;550 5.2.0 <[email protected]> - Recipient unavailable
The address does not exist, or is no longer active, at this provider. Remove it from your list and check for a typo in the local part or the domain.
Provider documentation
Postmaster & documentation
Comcast SMTP 550 questions
›What does an SMTP 550 error from Comcast mean?
The mailbox is unavailable and the message is rejected for good. This is the catch-all permanent failure, covering everything from an unknown address to an authentication failure to a reputation block, so the wording after the number is what actually tells you the reason. At Comcast, it is a permanent rejection, and the specific reason is in the enhanced status code and message text of each response below.
›How do I stop Comcast from returning SMTP 550?
Match your bounce text to one of the responses below and follow its guidance. Recipient-side wording means fix the address. Authentication or reputation wording (SPF, DKIM, DMARC, blocklist) means fix your sending setup, and check Comcast's postmaster documentation.