Apple Mail: The message could not be sent. You are not allowed to use sendmail utility.

[checkinindza]

Hi, I myself do not use a Mac computer, but we're having an issue with one of our clients.
They use a Mac and the included Mail app. We have disabled usage of sendmail utility on our server for some security concerns until we find a better solution, but we didin't anticipate that something like this would happen. Before our client had no issues sending mail, it worked fine. But after disabling the utility, the Apple Mail app throws this error:
"The message could not be sent. You are not allowed to use sendmail utility"
Both IMAP and SMTP settings are definitely fully correct. In theory, it should skip the sendmail utility and send mail right through our SMTP server, but for some reason it doesn't.
Are we missing something or we just lack the understanding on how macOS Mail app works?

 

[OldSchool]

What security concerns are you having with the sendmail utility. I hope that you have notified your other clients that you are doing this and so they don't jump on Apple Communities and start asking what is wrong with their email no going anywhere. It is this sort of thing that I think Microsoft has done and why Mail isn't working that well for Apple users if at all.