550 5.1.1: Recipient address does not exist
5.1.1 means the recipient address does not exist: a typo, a deleted account, or a guessed address. Gmail says "the email account that you tried to reach does not exist"; Microsoft calls it "bad destination mailbox address". It is a hard bounce. Remove the address and never retry it.
The exact messages
Gmail / Google Workspace
550 5.1.1 The email account that you tried to reach does not exist.
Microsoft 365 / Exchange Online
5.1.1 Bad destination mailbox address
Any mail server (RFC standard meaning)
5.1.1 Bad destination mailbox address
Why it happens
- A typo in the address.
- The person left the company and the account was removed.
- The address was guessed from a name pattern and never existed.
How to fix it
- Remove the address from every list and sequence.
- Verify lists before sending; a high hard-bounce rate damages sender reputation.
If you send cold email
Hard bounces are one of the fastest ways to burn a cold email domain. Verify every list before it goes into a sequence, and pause any campaign whose bounce rate climbs above a few percent.
How WarmHawk handles it
A hard bounce marks that lead as bounced and stops its sequence, and every bounce counts toward the mailbox’s bounce-rate breaker: at 5% over at least 20 sends, WarmHawk pauses the mailbox before a stale list can damage the domain. How WarmHawk works →
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Related bounce codes
- 5.1.10Recipient not found (Microsoft)
- 5.4.1Relay access denied / recipient address rejected
- 5.2.1Recipient account inactive or disabled
Sources, checked 2026-09-29: Google Workspace: Gmail SMTP errors and codes · Microsoft Learn: NDRs and SMTP errors in Exchange Online · RFC 3463: Enhanced Mail System Status Codes. Have a different bounce? Paste it into the decoder →
Questions
5.1.1 questions
What does 5.1.1 mean?+
5.1.1 means the recipient address does not exist: a typo, a deleted account, or a guessed address. Gmail says "the email account that you tried to reach does not exist"; Microsoft calls it "bad destination mailbox address". It is a hard bounce. Remove the address and never retry it.
Is 5.1.1 a temporary or permanent error?+
Permanent. The message will not be retried, and sending it again unchanged will fail the same way until the cause is fixed.
How do I fix 5.1.1?+
Remove the address from every list and sequence. Verify lists before sending; a high hard-bounce rate damages sender reputation.