Search an SMTP or enhanced status code and see what it means and what to do.
The recipient address does not exist or was rejected.
Next step: Remove the address and add it to suppression.
The mailbox in the address does not exist.
Next step: Validate addresses before sending.
The receiving server is temporarily unable to accept mail.
Next step: Retry later. Reduce rate for that provider.
The recipient mailbox is full.
Next step: Retry later; stop if it persists.
The message was rejected, often for policy or reputation.
Next step: Check authentication and blocklist status.
Rejected by policy, commonly authentication or reputation.
Next step: Verify SPF, DKIM and DMARC alignment.
The provider is throttling your mail.
Next step: Slow down and keep warming up.
Create an account, add your domain and send your first email.