Bounce "550 5.1.1": the address does not exist

What it means

You get an error message back (a "bounce") with a line such as:

550 5.1.1 The email account that you tried to reach does not exist

Other wordings include User unknown, Recipient address rejected or No such user. The receiving server is telling you the mailbox does not exist on its side. This is a hard bounce: trying again will not help.

Common causes:

• A typo: jonh@example.com instead of john@example.com, or gmial.com.
• The person left the company and the mailbox was deleted.
• An old address on a mailing list that has not been used for years.
• A fake address typed into a signup form.

Why it matters

One bounce is harmless. But if many of your emails go to addresses that do not exist, Gmail, Outlook and others see it as a sign of a poorly kept or bought list. Your reputation drops and your emails to real people start going to spam. Try to keep hard bounces under 2% of each campaign.

How to check

• Read the bounce. It names the address that failed and the server that refused it.
• Check the domain: type the recipient's domain at /domain-scanner. If it has no MX record, the whole domain cannot receive email, which is a different problem.
• Make sure the problem is not yours: if every email you send bounces with 5.1.1, even to valid addresses, check that you are not sending to a wrong domain by mistake (for example an autocomplete error).

How to fix it

1. Check the spelling of the address, letter by letter, and fix obvious domain typos.
2. Ask the person for their current address by another channel (phone, website, LinkedIn) if they are a contact you need.
3. Remove the address from your mailing lists. Most newsletter tools do this automatically after a hard bounce: do not re-import those addresses later.
4. Protect your signup forms. Use double opt-in so only confirmed addresses join your list, and add a simple check for common typos.
5. Clean old lists before sending to them. Addresses that have not received anything from you for a year or more are the riskiest.

If the address is in your own domain

If people tell you their emails to you@yourdomain.com bounce with 5.1.1, the mailbox or alias is missing on your own mail service. Create it, or set up a catch-all or alias in your provider's admin panel.

Check that it is fixed

After cleaning, watch the bounce rate of your next campaigns: hard bounces should drop close to zero. For a single contact, send one email to the corrected address and confirm that no bounce comes back.