This is an interesting bug discovered and evidently acknowledged by Microsoft: when using a non-Exchange, non-Gmail account like Hotmail, if you forward an email it sends itself as an attachment (*.eml) to the recipient instead of inline. See other reports here.
Microsoft has apparently agreed to look into the matter (acknowledged in a @winphonesupport tweet), which is a good start.
For now, the solution is simple if not awkward:
- Select Respond, as normal
- Instead of Forward, choose Reply
- Delete the email address in the To: field
- Replace with address you want to Forward to
Evidently this problem existed on our old WinMo devices (we actually vaguely remember this) and perhaps even on Android, meaning this may not require a fix on our end but rather Microsoft's.
Update: Our own Tim Ferrill tells us it happens with his Exchange 2003 server as well.
Thanks, anonymous, for the tip!