We use a third party system for sending bulk emailings. A strange thing occurred recently. We send multipart emails and they normally show up in our company's inboxes as HTML but within the past month one specific mailing started showing up in our inboxes as text with an HTML attachment. I noticed that the header previously had a content-type of multipart/alternative but it is now coming through as multipart/mixed. This is only a problem internally within my office. When we send messages to gmail, hotmail, etc. the header has a content-type of multipart/alternative and shows up as HTML. Even stranger, the emails show up fine in other company inboxes outside of my office (multipart/alternative).
My office uses MS Exchange Server but the administrator is not able to provide me with a reason for the recent change in content-type. What could be causing the content-type to change from multipart/alternative to multipart mixed.