Common causes
- The receiving mail server is offline or overloaded.
- A firewall or network block between the two servers.
- A DNS problem pointed to the wrong or an unreachable address.
- Your own network was congested.
How to fix it
- Let your server retry; most timeouts clear within hours.
- Test whether the recipient's mail server answers from another network.
- Check that your outgoing port 25 is open if you send directly.
- If it never works for a domain, treat it as unreachable.
Frequently asked questions
Does a timeout mean the address is invalid?
No. It means the server could not be reached. The address may be fine.
How does Mailcut handle servers that never answer?
If a domain has no mail server that accepts connections at all, the address is reported as Invalid; if it is only slow, it is reported as Unknown so you can retry.