Permanent (5xx)
550 5.1.1
Mailbox does not exist
The recipient address doesn't exist on the receiving server. This is the single most common hard bounce.
Common causes
- Typo in the recipient's email address.
- The account was deleted or never existed.
- Using a stale email list.
What to do
- →Never retry - suppress the address immediately, this won't succeed later.
- →Add real-time syntax + MX validation at signup to catch typos before they ever become a send.