The recipient's server says that address does not exist.
WHAT IT MEANS
The address is wrong, has been closed, or the domain has no mailbox by that name.
CHECK
The spelling, character by character, including the domain Whether the person has left that organisation Whether the domain itself 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 that address bounces, which is often correct.
Do not set up a catch-all to solve this. Catch-alls accept mail to every possible address and attract enormous volumes of spam.
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 from any list rather than retrying them.
