In brief
- Gmail: "The email account that you tried to reach does not exist." Microsoft: "Bad destination mailbox address".
- It is a permanent address failure, not a problem with your reputation or your DNS.
- Most frequent cause: a typo or an address that has become invalid. A few special cases are worth knowing.
- Remove the address from your list after the first refusal. Do not retry it.
What the code says exactly
In 5.1.1, the first 5 signals permanent failure, the 1 the "address" category and the last 1 the detail "unknown destination mailbox". Not all servers use it the same way: Microsoft distinguishes 5.1.1 (incorrect or nonexistent address) from 5.1.10 (recipient not found) and from 5.5.0 for hotmail.com or outlook.com addresses.
Do not confuse it with a full mailbox. Google returns a 552 5.2.2 when the recipient storage is full: the address exists, it just cannot receive for now. A 5.1.1 is never resolved by waiting.
The causes, and how to tell them apart
Typo or space in the address
How to recognize it: The address in the refusal is visibly incorrect: misspelled domain (gmial.com), space, comma or accented character in the local part.
What to do:
- Correct the address if the mistake is obvious. Google advises checking for typos and extra spaces.
- When importing a file, clean the spaces at the beginning and end of each cell before sending.
Deleted mailbox or person who left
How to recognize it: The address is written correctly and matches a plausible format (firstname.lastname@company.com), but the server says it is unknown. This is typical of an employee who left the company.
What to do:
- Remove the address from your prospecting list.
- Look for the new contact at the company rather than retrying the old one.
Old list or guessed address
How to recognize it: The 5.1.1 errors arrive in batches from the first sends on a purchased list, an old list or a list built by guessing address formats.
What to do:
- Stop sending to this list. Yahoo specifies that many unknown recipients look like an open relay and can lead to a broader block.
- Check the addresses before sending. Fluenzr's list verification checks the MX record and disposable domains: it rules out invalid domains, but does not by itself prove that a specific mailbox exists.
Recipient forwarding rule pointing to an invalid address
How to recognize it: The address shown in the refusal is not the one you wrote to. This is a sign that a forwarding rule on the recipient side sends the message to a broken address.
What to do:
- Compare the address in your Sent folder with the one in the refusal. If they differ, warn the recipient through another channel: their forwarding rule needs to be fixed.
Outdated Outlook autocomplete cache
How to recognize it: You write from Outlook to someone whose address has not changed, but whose mailbox has been moved (for example from on-premises Exchange to Exchange Online).
What to do:
- Delete the recipient's entry from Outlook's autocomplete list, then retype the address. Microsoft documents the procedure for Outlook and for Outlook on the web.
Refusal for a message you did not send
How to recognize it: Your Sent folder contains no message matching the refusal. Either it is a stray bounce, or your account is being used without your knowledge.
What to do:
- If you find no suspicious message in your sent items, it is a stray bounce (a spammer spoofed your address). You can ignore it.
- If unknown messages appear in your sent items, change the password, scan your devices and check the forwarding rules and delegations of the mailbox.
Check your DNS
Before concluding that the address is dead, two domain checks rule out the causes that are not about the mailbox.
Does the recipient domain receive mail?
dig +short MX recipient-domain.com
Expected result: At least one line with a mail server. No answer: the domain has no mail service, all sends will fail.
Find the address that was actually refused
Open the refusal message and read the "Final-Recipient" or "Delivery has failed to these recipients" line
Expected result: The address that failed. Compare it character by character with the one in your list.
Check your whole domain at once
The free diagnostic reads your SPF and your DMARC and looks for a DKIM signature, with no account. It is the fastest way to find out whether authentication is the problem.
Test my domain nowWhat not to do
- Retrying the same address several times: the failure is permanent.
- Leaving refused addresses in the list for the next campaign.
- Waiting for it to go through: unlike a 4xx, a 5.1.1 does not resolve with time.
Frequently asked questions
What does 550 5.1.1 mean?
The recipient address does not exist on the receiving server. At Gmail, the text is "The email account that you tried to reach does not exist". At Microsoft, "Bad destination mailbox address".
Does 5.1.1 hurt my sender reputation?
Repeated refusals for nonexistent addresses show a poorly maintained list. Yahoo lists them among the causes of a broader block ("excessive unknown recipients"). An isolated refusal is normal, a high rate is not.
What is the difference between 5.1.1 and 5.1.10?
Microsoft uses 5.1.1 for an incorrect or nonexistent destination address, and 5.1.10 when the SMTP address lookup does not find the recipient. In both cases, the address should be removed from the list.
Does a full mailbox give a 5.1.1?
No. Google returns a 552 5.2.2 for full storage. The address exists: a new attempt later may go through.
Official sources
- Google: Gmail SMTP errors and fixes
- Yahoo: error codes for senders
- Microsoft: fix error codes 550 5.1.1 through 5.1.20 in Exchange Online
- Microsoft: non-delivery reports and SMTP errors in Exchange Online
Messages and fixes taken from the official documentation on October 21, 2026. Providers sometimes change their wording: if there is a discrepancy, the official page prevails.
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