The recipient address does not exist.
WHAT IT MEANS
The receiving server says there is no such mailbox.
CHECK
The spelling, character by character, including the domain Whether the person has left that organisation Whether the domain is correct, particularly .com versus .com.ng
IF IT IS AN ADDRESS ON YOUR OWN DOMAIN
Someone is sending to an address you have not created.
Either create the mailbox, add a forwarder, or accept that mail to it bounces, which is frequently correct.
Do not create a catch-all to solve this. Catch-alls accept mail to every possible address and attract enormous spam volumes.
IF IT STARTED SUDDENLY FOR A WORKING ADDRESS
The recipient's mailbox may have been deleted, or their domain may have lapsed.
Contact them another way.
IN BULK SENDING
Repeated bounces to unknown addresses damage your sending reputation.
Remove invalid addresses rather than retrying them.
