Polly,
Using Plain Text vs RTF has dramatic impact to how the message is coded, and then for certain receiving email clients, such as Outlook and Outlook Express may be problematic, especially if the user is not really skillful in such matters. To see the difference, observe messages sent in each format by opening them in the Sent mailbox, clicking on View in the menubar, placing your cursor on Message in the resulting pull-down and choosing Raw Source. With those in RTF, you will note lots of HTML looking code, and if an attachment is present, the receiving email client seems to miss the header that is in front of every attachment.
Those particular MS email clients don't handle Rich Text Format well, and if they send with it, it is in a proprietary version that other email clients have trouble with.
This is an extremely well-tested issue, and you should first try using Plain Text.
Ernie