Do recipients of your emails, seemingly out of the blue, complain about a mysterious attachment called “winmail.dat” (of the even more mysterious content type “application/ms-tnef”), which they cannot open, no matter what they try? Do files you attach disappear in that winmail.dat moloch? Does winmail.dat show up for some but not all recipients of your messages?
It turned out that this was the case for several of our users using Outlook after migrating from Exchange to Dovecot.
Solution (must be performed by the sender):
- On the Tools menu, click Options, and then click the Mail Format (or “Send”) tab. Under the Send tab is the message format list, select Plain Text or HTML (NOT Rich Text Format), and then click OK.
- Then go through all contacts in Addressbook, and check that the relevant option is not set to «Rich Text Format» as well.