550 5.1.1: The recipient mailbox does not exist

At a glance

• Type: permanent failure (class 5, RFC 3463 §2). The sending server does not retry; sending the same message again fails the same way until the cause is fixed.
• Basic code 550: “Requested action not taken: mailbox unavailable (e.g., mailbox not found, no access, or command rejected for policy reasons)” (RFC 5321 §4.2.3).
• Enhanced code 5.1.1: “Bad destination mailbox address” in the IANA registry (RFC 3463), in the group addressing status (RFC 3463 §3.2).

The receiving server says the mailbox you wrote to does not exist. The domain is fine, but nobody by that name has an account there, so the message will not be delivered to this address.

What it means

A 550 reply is a permanent refusal: the server will not take the message and the sending server should not retry (RFC 5321 §4.2.1). The enhanced code 5.1.1 narrows the reason down. RFC 3463 §3.2 defines X.1.1 as a bad destination mailbox: for an Internet address, the part to the left of the @ is invalid. The same section says this code is only useful for permanent failures, so a temporary 4.1.1 is uncommon; some servers send one while they cannot yet confirm the address.

Common causes

• A typo in the local part, such as jhon@ instead of john@.
• The person left the company or closed the account, and the mailbox was deleted.
• The address was made up, mistyped on a sign-up form, or guessed from a naming pattern.
• An old entry in an address book or autocomplete list that points to a mailbox that is gone.

How providers use it

• Gmail returns 550 5.1.1 when the account does not exist and suggests checking the address for typos and stray spaces (Google's SMTP error reference). A typical line reads 550 5.1.1 The email account that you tried to reach does not exist.
• Microsoft 365 uses 5.1.1 for a bad destination mailbox address. Its NDR reference names a mistyped address, an address that does not exist, and a stale cached autocomplete or nickname entry as reasons (Exchange Online NDR reference). Exchange Online also reports unknown recipients as 550 5.1.10, and Outlook.com as 550 5.5.0.
• Yahoo reports invalid recipients with 553 or 554 permanent errors and asks senders to remove the address (Yahoo SMTP error codes).

How to fix it

1. Read the address in the bounce character by character. One wrong or missing letter is enough to cause this bounce.
2. If you have another way to reach the person, ask for the correct address. Do not guess a new spelling and send to it.
3. In Outlook, delete the old autocomplete entry so the bad address is not offered again.
4. If you run the receiving domain and the mailbox should exist, check that the account and its aliases are set up.

What to do with the address

Stop sending to it. Mark the contact as a hard bounce in your list or CRM and suppress it from future campaigns. If many addresses in one list return 5.1.1, the list itself is the problem: check it before the next send with bulk email verification and read the list cleaning guide.

Related codes

• 550 5.1.10: Recipient not found (Microsoft 365)
• 550 5.5.0: Mailbox unavailable at Outlook.com or Hotmail
• 553 5.1.3: The recipient address is not valid syntax
• 550 5.2.1: The mailbox is disabled and takes no mail

See every bounce code in the dictionary.

Many bounces from addresses that do not exist can be avoided: check an address with our free email checker, or clean a whole list before you send.

How we wrote this: drafted by the email-validation.org team with the help of AI writing tools, from our product documentation and the sources linked in the text.