I've been getting notifications from qmail whenever an automated email is sent to particular emails, that the email could not be delivered.
These email addresses are from a domain that we used to host and we don't anymore.
So we can't send emails to these addresses from our server anymore.
We were originally told that our server was looking at old MX records to send the emails.
We have since updated the MX records, but the problem is still recurring.
I've tried using the dig command "dig mx <domain name>" and this shows the updated MX records.
I don't know how our server is trying to send emails to these addresses...