Common causes
- Your sending IP is on a blocklist.
- The server does not relay mail for that domain and the recipient is not hosted there ("relay access denied").
- The message was classified as spam or malware.
- Your connection came from a network the provider blocks.
How to fix it
- Read the text after the code; it normally says which of these it is.
- Check your IP and domain on blocklists and fix the cause before asking for removal.
- If it says relay denied, the address is probably wrong for that server.
- Send a plain, short test message to see whether content is the trigger.
Frequently asked questions
Is "relay access denied" the same as an invalid address?
Often yes: it means that server does not serve that domain for that recipient. Mailcut treats it as Invalid only when every mail server of the domain gives the same answer.
Why is 554 so vague?
Because many different problems share it. The text after the code is what tells you which one you have.