This issue is caused when older versions of Outlook are sending attachments while using the RTF formatting. It can be solved client side by ensuring the sender sends in HTML format. (or better yet, plain text!) That said. We need a server side fix for those clients who insist that, "It worked when we used Gmail!" I've found mimedefang, and I think it may very well be applicable here. Unfortunately though the walk through I've found is for another mail collaboration platform. Other references with Postfix seem to be a little incomplete or expect a good working knowledge of Postfix to deploy. https://forum.proxmox.com/threads/tnef-winmail-dat-decoding.51374/ Would anyone have any tips or suggestions for: a) installing/configuring mimedefang in an ISPConfig setting? or b) an alternative to mimedefang that would accomplish similar results? I did come across another post where it was explained that amavisd is used by ISPConfig and that it could possibly do it?