N. Miller said:
The error message says, "while talking to smtpserv.absamail.co.za". Unless
'smtpserv.absamail.co.za' serves you, then your server did not reject this.
As a rule, SMTP message submission servers will not reject email for a "User
unknown"; unless the server is badly configured. The message submission
server should not know the state of existence of a "RCPT TO:" email address.
A domain gateway (MX) SMTP server may reject email for a "User unknown", if
it knows the state of existence of the "RCPT TO:" email address. It may also
reject any incoming email for any reason, and "lie" (due to error, or
misconfiguration by the administrator) about the reason.
If you can send an original email to that address, but a reply is rejected,
there may be something about how the reply is created by your client that
the gateway mail server doesn't like. You could try replying without
quoting, to see if it is something about the quoting that is triggering the
reject.
It is also possible that the original sender screwed up and entered an
invalid or incorrect e-mail address in the Reply-To header. When
sending e-mail to this person, you are using their correct e-mail
address. When replying to their e-mail, you are, by default, using the
Reply-To e-mail address that the sender specified, if they specified
one.
You sending new e-mail to this person has you using their correct e-mail
address that you recorded in your contact records.
You replying to this person's e-mail is using their Reply-To header to
specify what e-mail address to use as the recipient in your reply. If
they got it wrong, you reply to that wrong e-mail address.
So when you reply, try the following: do a reply but wipe the To field
to remove their e-mail address, and then use their e-mail address from
your contacts list. If it goes through, tell them to correct their
Reply-To field in the e-mail account they defined in their e-mail client
as it is invalid. If it doesn't go through, neither will new e-mails
sent to them as they have a problem at their end.
Although you think the e-mail address is correct, how do you know that
person still has an active account at that domain? They might've closed
it. It might be a freebie account and they abandoned it for too long so
it got disabled. They or their e-mail provider might've closed that
account.