Common causes
- The receiving server is under maintenance.
- The mailbox is locked while it is being moved or repaired.
- The provider limits mail to the mailbox for a short time.
- A short network or storage problem on the receiving side.
How to fix it
- Wait and retry; most cases clear within hours.
- Do not mark the address invalid on one temporary answer.
- If it lasts more than two or three days, treat the address as unreliable.
- Keep a note of repeated deferrals for the same domain.
Frequently asked questions
Is the address bad?
Not on this answer alone. A 4 in the code means temporary. Look at the pattern over several sends.
What does Mailcut do with it?
The address is reported as Unknown, and you can re-check unknown results once at no cost after the bulk finishes.