4.4.1: no answer from the receiving server

What it means

4.4.1 is written by the sending side: its attempt to open a connection to the recipient's mail server was not answered, because that server was busy, down or unreachable. RFC 3463 calls it useful only as a persistent temporary error, so the message stays queued and is retried, and it comes back to the sender only if the retry period runs out. You will also see 5.4.1 in bounces, but some providers use that form with meanings of their own: Microsoft's Exchange Online, for example, sends 550 5.4.1 to refuse recipients its directory does not know. Read the text, and the provider's own page, whenever you meet 5.4.1.

Common causes

• The receiving server is down or restarting.
• A firewall blocks inbound connections on port 25, or the MX record points to an address that does not answer.
• The receiving server is too busy to accept new connections.
• Network trouble between the two servers.

How to fix it

1. 01Wait: the sending server retries on its own.
2. 02Run an MX lookup on the recipient domain and check that the listed hosts resolve to addresses.
3. 03If you run the sending server, test a connection to the recipient's MX host on port 25 from it.
4. 04If the message stays queued, tell the recipient so their administrator can check the server.

Sources

What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.

• RFC 3463, section 3.5: Network and Routing Statusrfc-editor.org
• RFC 4468, section 6: Response Codes (BURL)rfc-editor.org
• Microsoft Learn: Fix NDR error 550 5.4.1 in Exchange Onlinelearn.microsoft.com
• IANA, SMTP Enhanced Status Codes registry (Enumerated Status Codes)iana.org

Further reading

• MX lookup