What is the difference between 5.1.1 and 5.1.2?

5.1.1 means the domain is fine, but the specific user doesn't exist. 5.1.2 means the entire domain doesn't exist or isn't set up for email.

Could this be a temporary DNS issue?

If a domain's DNS is temporarily down, it usually results in a 4xx deferral. A 550 5.1.2 means the sending server conclusively determined the domain is invalid.

← Error library

550 5.1.2 Bad destination system address

permanent Hard Bounce

Meaning

The destination domain specified in the email address does not exist or cannot accept mail.

Common causes

The domain part of the email address (after the @) has a typo.
The recipient's domain has expired or their DNS MX records have been removed.
The domain is a fake address provided by a user during signup.

How to fix

Remove the address from your mailing list.
Check for obvious domain typos (e.g., @yahooo.com instead of @yahoo.com) and correct them if you have a process for it, otherwise suppress.
Ensure your system validates domains at the point of capture.

Provider notes

Gmail. Often uses 553 5.1.2: 'We weren't able to find the recipient domain.'

Example bounce

550 5.1.2 Bad destination system address.

FAQ

What is the difference between 5.1.1 and 5.1.2?: 5.1.1 means the domain is fine, but the specific user doesn't exist. 5.1.2 means the entire domain doesn't exist or isn't set up for email.
Could this be a temporary DNS issue?: If a domain's DNS is temporarily down, it usually results in a 4xx deferral. A 550 5.1.2 means the sending server conclusively determined the domain is invalid.