Receiving winmail.dat files from dad—he’s using Apple Mail, not Outlook or Gmail

Before I start a firestorm, YES, I’ve searched for this answer on the Apple support forum and other sites.

Just recently, my father’s email attachments have shown up as winmail.dat files. He is on a fairly new Mac, not using Catalina yet, but the latest OS before that, and he’s using Apple Mail. Other support posts talked about this issue with a Gmail sender or people using Outlook. He is NOT.

Until recently, his emails were fine. The image below shows what came in to my Apple Mail app today when he sent a PDF. He is sending by dragging the file into his Apple Mail window, the way he’s always done it. I cannot open these files, and I’m uninterested in a solution that requires me to use another program to open them. I receive regular files from others just fine...I’ve isolated the problem to my dad’s email messages only.

I do know that when he sends files this way, the ARE viewable by recipients on a Windows PC using Outlook, and there’s no problem. It only seems to be having a problem with others on Macs or iOS devices.

The one other possible complicating factor is that because his email was inundated with junk and phishing emails, so GoDaddy recommended we set up his email on Exchange so he could take advantage of their spam filtering. I’m wondering if Exchange is to blame...or if there’s a setting we can use to send the files normally. I did see another article about turning off RTF when using Outlook, and I’ll be on his system tonight and see if this is possibly the culprit. (Isn’t Microsoft ALWAYS the culprit??😁)

I’d be appreciative if anyone else has had this problem and could offer some suggestions.

(And yes, that’s 140,000 unread messages...bad, bad Kitty. My @mac.com address is an early one that is often “claimed” by new Apple users, and they sign me up for all sorts of junk.)