If you are getting "550 5.1.1" back from Gmail or Outlook, the receiving server is saying the mailbox you are trying to reach does not exist. This is a hard bounce: the address is dead, and retrying will not help. It is usually a list problem, not a domain problem. Here is how to handle it so it stops dragging your sender reputation down.
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Run the free checkThe first digit, 5, means a permanent failure. The enhanced code 5.1.1 means the mailbox you are trying to reach does not exist on the receiving server. In plain terms, the address is dead. The server has no such user, and no amount of retrying will change that.
1. A typo in the address. A missing or extra character, or the wrong domain, produces a 5.1.1 even when the person is real.
2. The recipient left. The person changed jobs or left the company, and their mailbox was deleted.
3. The address was never valid. The address was scraped or guessed and never actually existed.
4. The receiving server has no such user. Even a correctly formatted address can 5.1.1 if the mailbox was removed.
1. Verify before you send. Check each address for typos and confirm the domain is the one you intend to reach.
2. Remove hard bounces. The moment you get a 5.1.1, drop the address from your list so you stop hitting it and inflating your bounce rate.
3. Validate your list. Run your list through a validation tool before a campaign to catch dead addresses up front instead of after the bounce.
4. Keep your sending domain healthy. A clean SPF, DKIM and DMARC setup means your valid recipients actually receive your mail, and the bounces you do get are real, not a configuration failure.
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