Common causes
- The person never clears their inbox.
- A shared or role mailbox filled up with automated mail.
- An old account that nobody reads any more.
- A small storage limit on a free or trial mailbox.
How to fix it
- Retry once or twice over a few days; some people free space.
- If it keeps failing, treat the address as inactive.
- Contact the person another way and ask them to clear space.
- Keep large attachments out of your messages; send a link instead.
Frequently asked questions
Is a full mailbox a good address?
The mailbox is real, so it is not an invalid address, but nobody is reading it. For marketing lists it is usually better to drop it.
How does Mailcut report it?
Mailboxes that answer as full are reported as valid with a note, because the address exists; you decide whether to keep it.