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4.4.1 deferral: no answer from host

4.4.1No answer from hostTemporary: the sender retries

Example
dsn=4.4.1, status=deferred (connect to mx.example.com[203.0.113.10]:25: Connection timed out)

What it means

RFC 3463 defines X.4.1 as "No answer from host". Your sending server tried to open a TCP connection to the recipient’s mail host and either got no reply (a timeout) or an outright refusal. No SMTP conversation took place, so the recipient’s server never saw your message. The example is a Postfix log line for the timeout case.

Because the remote side never answered, the code is generated by your own MTA, not received from the recipient. It shows up in delay notices and, after the queue lifetime, in the final bounce. It differs from 4.4.2 (the connection opened and then broke) and 4.4.3 (DNS could not be resolved in the first place).

The temporary 4.4.1 is by far the usual form, since hosts come back. A 5.4.1 is rarer. Microsoft 365 also returns 550 5.4.1 for its own recipient rejections, which has a separate page.

Likely causes

  • The recipient’s mail server is down or overloaded.
  • A firewall on either side blocks TCP port 25, for example a cloud provider or ISP blocking outbound port 25 from your server.
  • An MX record points at an old IP address that no longer runs a mail server.
  • Your server tries IPv6 first and the recipient’s AAAA address is not reachable from your network.

How to fix it

  1. Look up the recipient MX hosts and test port 25 on each one from your server, not just from your desk.
  2. If the connection times out to every destination, check whether your hosting provider blocks outbound port 25 and ask for it to be opened, or relay through a smart host.
  3. If only IPv6 fails, fix your IPv6 routing or prefer IPv4 for outbound SMTP (Postfix: `smtp_address_preference = ipv4`).
  4. If you own the recipient domain, confirm the MX targets resolve to the right addresses and the server listens on 25.
  5. Otherwise let the queue retry and inform the recipient of their outage.

Checks that apply

  • TCP Port Test Tests whether port 25 on the recipient host accepts connections, which is exactly what a 4.4.1 says failed.
  • MX Lookup Shows the MX hosts and addresses your server is trying, revealing stale or wrong targets.
  • Traceroute Traces the route to the mail host to find where packets stop.
  • AAAA Record Shows whether the MX host has IPv6 addresses, a frequent factor when only some attempts time out.
See the example diagnosed

The classification runs without network lookups and we never store your paste.

FAQ

Did the recipient reject my message?
No. Their server never answered, so it could not reject anything. The connection itself failed.
Why does it work from my laptop but not my server?
Many cloud and residential networks block outbound port 25 from servers. Test from the server that actually sends the mail.

All 191 bounce codes