We have written an Outlook Add-in and it appears that when you add user properties to a MailObject while the user is composing it, and you then encrypt and sign it and the message is Sent, the message will end up getting sent as TNEF, despite imploring outlook to do otherwise through various settings and so on as described here: http://www.slipstick.com/problems/outlook-is-sending-winmail-dat-attachments/
So one of our clients has a contact who insists on encrypted communication and therefore our client now has an issue with this contact. Either they cannot use our Add-in to its full potential (having to avoid the functionality that adds those User Properties), or their contact complains about receiving mail with "winmail.dat" attachments.
I have since established a communication with our client's contact, and I am trying to establish what e-mail client they are using, and one thing I'm going to try is see whether they would be open to the idea of moving to another e-mail client that is TNEF capable, even if it's not Outlook. But my Google-Fu is failing me. I've googled "TNEF capable email clients" and many variations thereof "that can use" "able to" ... etc etc etc. Nothing gives me the result I am looking for, a simple list of non-outlook email clients that have native capability for handling TNEF e-mails they receive. Plenty of articles of tools to allow users to decode the winmail.dat attachments manually, sure, but no simple list of natively capable e-mail clients.
If anybody can help me with this one, it would be greatly appreciated.
Eudora used to support TNEF. Otherwise Outlook is the only one to the best of my knowledge.