Common causes
- Recipient doesn’t exist
- Blocked IP or domain reputation
- SPF/DKIM/DMARC failures
Fixes
- Verify the address and mailbox quota.
- Check your DNS records.
- Send a plain‑text test without links.
- Contact Support with full headers if persistent.
Codes
- 550: Mailbox unavailable
- 553: Authentication required
- 554: Transaction failed (policy block)