Sounds like some kind of spam filter or other kind of content filtration system. The problem is likely at the other end (the destination of the message) since typically outgoing messages aren’t scanned.
It could be something as simple as a “bad” word that forms part of another word (for example, I remember seeing an email from a friend classified as spam because we were discussing Rush songs, and the email in question contained the song title “The Analogue Kid” – the first part of the second word was interpreted as being part of a spam email).
Since you mentioned that the email was discussing medical advice, it’s possible that some of the more spam-prone drug brand names were in the text of the message (Viagra, Vioxx, Cialis, etc., etc.). Some spam filters are set just a tad too strict.
I could be wrong, but that’s what I think’s happening. Review the message in question with these thoughts in mind.