Common causes
- Your sending IP or domain is on a blocklist or has a poor reputation.
- SPF, DKIM or DMARC is missing or failing.
- The content or links look like spam to that provider.
- The recipient organisation only accepts mail from approved senders.
How to fix it
- Read the text after the code: it usually names the rule, and often links to the provider's postmaster page.
- Check your authentication records and fix any failure first.
- Look up your IP and domain on common blocklists and ask for removal if listed.
- Slow down, send to engaged people only and try again later.
Frequently asked questions
Does this mean the address is invalid?
Not necessarily. This answer is about you as the sender, not about the mailbox. Treat it as a deliverability problem, not a list problem.
What does Mailcut show for it?
When the refusal is a policy answer we do not guess: the address is reported as Unknown or Accept-all rather than Invalid.