winmail.dat - any way to fix this issue from the mac (receiver) side?

acypher wrote:

Yes, thanks! But I'm still hoping for a fix to the Apple Mail.problem.

The Microsoft mail server involved here is misconfigured and is sending out a proprietary- and subject-to-change Microsoft message format message.

The sender of the message can override this setting, or the sending IT can fix this on the mail server, or you can use a tool to read the message.

Basically, this is an Outlook-to-Outlook-format mail message leaking out further than it should have.

https://support.microsoft.com/en-gb/topic/how-email-message-formats-affect-internet-email-messages-in-outlook-3b2c0536-c1c0-1d68-19f0-8cae13c26722

https://support.microsoft.com/en-us/topic/winmail-dat-sent-as-an-email-attachment-in-outlook-2007-and-2010-b72c81ac-db5e-b142-d43d-448efbc23cce

The “fix” here is with Microsoft; with the sender.

Or to switch to and use the Outlook client and its TNEF support, as the receiver.

Or use a third-party TNEF-decoding tool.

Here is a copy of the Apple HT201773 tech note.

TNEF's Enough is a macOS app designed to identify open winmail.dat files. Download it from the Mac App Store, launch it, drag the winmail.dat attachment onto it, or right-click the attachment and choose 'Open with' and TNEF's Enough. Once the application has opened it, you can export it in the correct file format.

Runs on Mac OS 10.11 to 10.15

On the problem Mac...

Are you running any VPN, Anti-Virus, or Cleaning apps?

Safe Boot from the HD, (holding Shift key down at startup), does the problem occur in Safe Mode?

Safe mode attempts to repair Disks & clears lots of caches, so if Safe Mode works try again in regular boot.

If you have no POP accounts & they’re all IMAP accounts then in Mail’s Mailbox menu choose Rebuild.

acypher wrote:

If it's a problem with the sending mail server, that doesn't seem to explain why two recipients with identical configurations are getting different results

Unless both recipients are sharing an email address, because the sender can select the sending behavior on a per-recipient basis. Here assuming Outlook is not in use by either recipient, too.

Do I need to have the sender send the email while I am in Safe Mode?

No.

There is one POP account and one IMAP account.

Danger with a POP account & Rebuild, is that normally POP accounts remove the eMail from the Server when downloaded & Rebuild will remove any POP eMails that are already downloaded, leaving you with Zero POP eMails.