Mail sent from a Mac (OS X Version 10.7.5 using Mac Mail, via Office 365 / Exchange) is appearing as it should when the mail consists of just text.
However, when there is an attachment added (in the case of my example a word document, but it's been the same for any file type) the email body text does not appear in the recipient's mail program (Outlook 2010 in this case) but is present as a .htm attachment - as is the signature added in Mac Mail.
If anyone's come across this or even better knows how to stop it from happening (My only thought so far was an encoding issue?), I'd be pleased to hear from you. There are a couple of open questions on this site which I think concern the same issue.