I have a user whose 9780 has suddenly started displaying every "m" in received emails as a "b" followed by a space. (Note, this is not a keyboard problem, this is a problem with the displaying of emails sent to him by other people.)

Apparently these same emails look ok in Outlook (we're on BES).

I checked that the language is set to English. What else should I check?

For this user, unless there are imbedded control characters I would try to reload the OS on the device. Do a backup, a wipe, then using some of the tools suggested in the FAQ section on the top of the forum page, do a reload. Sounds like a simple OS corruption. Why it occurred could be interference from an added app.