Employee payslip emails bouncing

I have an employee to which I can send direct emails and get responses but his payslips from MYOB are bouncing back. We have liaised with the employee and their email address is correct and they are able to receive emails from our work emails. The below is the message that we have received:

Sorry, email delivery was unsuccessful.

Diagnostic-Code: smtp; 550 5.5.0 Requested action not taken: mailbox
unavailable (S2017062302).

I have seen other threads for this but cannot add comments as they are closed.

Can someone from MYOB please check if his email has been blacklisted?