Comcast (Xfinity) · permanent error
550: sender domain has no A or MX record
Xfinity refused the message because the domain you send from has neither an A record nor an MX record in DNS. Add one and send again.
- Comcast
- Provider
- Permanent (5xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 550
- Provider
- Comcast (Xfinity)
- Temporary or permanent
- PermanentRefused for good: the message comes back to the sender, and resending it gets the same answer.
- Who has to act
- The sender. The fix is on the sending side: your domain, your server or your list.
- What the server says
550 Invalid sender domain
What it means
Xfinity requires every sending domain to have a valid A or MX record. If it can find neither for the domain you send from, it rejects the message with this 550. Without either record, replies and bounces to that domain have nowhere to go. Cases that cause it include a subdomain or a newly registered domain used for sending before its DNS was finished, and a typo in the sender address configured in an application.
Common causes
- The domain you send from has no MX record and no A record.
- The sender address has a typo, so its domain does not exist.
- A sending subdomain was set up for SPF or DKIM only, with no A or MX record.
How to fix it
- 01Look up the A and MX records for the exact domain in your sender address, including any subdomain.
- 02Add an MX record, or at least an A record, for that domain.
- 03Correct the sender address in the application if it is wrong.
- 04Send the message again once the records resolve.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
Related codes
- 451Sender domain could not be resolved.Yahoo could not confirm that the domain in your envelope sender or From address exists. As a 451 it is temporary; Yahoo can also return the same problem as a permanent 554.
- 550Mailbox unavailable.The receiving server permanently refused the message for this recipient: the mailbox does not exist, cannot be reached, or the server rejected the message by policy.
550 at other providers
- 550 [SC-001]Outlook.com policy block for content or reputation.Outlook.com rejected the mail for policy reasons tied to spam-like content or to the reputation of the sending IP or domain.
- 550 [SC-002]Outlook.com block for namespace mining.Outlook.com blocked the sending IP because it behaved like a namespace miner, checking whether addresses exist without sending mail.
- 550 [SC-003]Outlook.com block for an open proxy or relay.Outlook.com rejected the mail because the sending IP appears to be an open proxy or open relay.
- 550 [SC-004]Outlook.com block after complaints.Outlook.com placed a block on the sending IP because it received complaints about mail coming from it.
- 550 [DY-001]Outlook.com refuses dynamic IP addresses.Outlook.com rejected the mail because the sending IP is dynamic address space, which it generally does not accept mail from.
- 550 [DY-002]Outlook.com block for a likely compromised sender.Outlook.com rejected the mail because the sending server or computer appears to be compromised or infected.
Further reading
All 33 Comcast codesEvery code in the wikiPaste your own bounce