What it looks like in your logs
postfix/smtp[15220]: 3B71C2403A: to=<user@example.com>, relay=none, delay=30, status=deferred (connect to mx.example.com[198.51.100.25]:25: Connection timed out) dsn=4.4.1What does 4.4.1 mean?
RFC 3463 defines X.4.1 as no answer from host: the sending server tried to open a connection to the recipient's mail server and got nothing back. No SMTP conversation happened, so the receiver has not judged your message at all. Your MTA marks it as deferred and tries again later. Nothing about your message, content or reputation is involved at this stage.
Most 4.4.1 deferrals come from the receiving side: a server that is down, a firewall that drops traffic on port 25, or MX records that point to an address with nothing listening. A few come from your side, for example a cloud host that blocks outbound port 25 by default. If every destination returns 4.4.1, look at your own network first.
You will see 4.4.1 in your own MTA logs and in Microsoft 365 NDRs, where Exchange Online reports "451 4.4.1 connection timed out" or similar when it cannot reach a destination. Large consumer providers are rarely the source because their MX hosts are highly available.
Soft or hard bounce?
| Soft bounce (4xx) | Hard bounce (5xx) | |
|---|---|---|
| Nature | Temporary | Permanent |
| Sender behaviour | Queue and retry with backoff | Stop, suppress the address |
| Reputation risk | Low, unless it repeats for days | High above a 2% bounce rate |
| 4.4.1 | ✓ this code |
Common causes
- The recipient mail server is offline or restarting.
- A firewall at the recipient blocks or drops inbound port 25.
- The recipient MX record points to a wrong or retired IP address.
- A network routing problem between your server and the destination.
- Your own host or cloud provider blocks outbound port 25.
How to fix 4.4.1
- Let the MTA retry on its normal schedule; many outages clear within hours.
- Check the recipient domain's MX records with the MX lookup and confirm they point to live hosts.
- Test a connection to port 25 on the MX host from your server to see whether it answers.
- If every domain times out, check whether your provider blocks outbound port 25 and request it be opened or use a relay.
- If a single domain times out for more than a day, contact the recipient by another channel.
4.4.1 questions
What does 4.4.1 no answer from host mean?
It means your mail server could not connect to the recipient mail server, usually because of a timeout. No SMTP exchange happened. It is temporary, so the message stays in the queue and is retried until it goes through or the retry period expires.
Is 4.4.1 a problem with my email server?
Usually not. It is most often an outage or firewall at the recipient, or a bad MX record. If you see 4.4.1 for many unrelated domains at once, check your own network, because your host may block outbound port 25.
How is 4.4.1 different from Microsoft 5.4.1?
Officially they share a meaning, but Microsoft reuses 5.4.1 for recipient rejections with access denied. A 4.4.1 is a network timeout that will likely clear. A Microsoft 5.4.1 is a permanent refusal from the recipient tenant. Handle the two very differently.