Documentation
5.1.2: the recipient's domain could not be found.
There is no mail server to deliver to, so the message went nowhere.
What it means
A 5.1.2 reply means the receiver (or the network on the way) could not find a mail server for the domain after the @. The wording is usually "host not found", "domain not found" or "no MX". A 5.1.10 reply is a related case: the domain publishes a null MX record, which is its way of saying it accepts no mail at all.
Typical codes and wording
550 5.1.2, 5.1.10 ("host not found", "no MX", "null MX")
Common causes
- A typo in the domain part of the address (gmail.con, hotmial.com).
- The domain has expired, was never registered, or has no mail records.
- The domain deliberately accepts no email (a null MX, used by websites that never receive mail).
What to do
- Look at the part after the @ and compare it with the person's real address.
- If it is a typo, correct it and send once to the right address.
- If the domain really has no mail server, the address cannot receive mail; remove it.
- Do not retry the same address unchanged.
What Mailtruss shows
Titled "Recipient domain could not be found" (5.1.2) or "Recipient domain does not accept email" (5.1.10). Both are permanent, so the address is added to your suppression list.
Is it safe to retry?
No, not until the address is corrected.