As you have no control over the way in which the recipients view email
messages, as the astute ones will have Outlook configured so that messages
are read as plain text, the only way to ensure that they see the "message"
as you wish is to send it as an attachment.

See the article "Mail Merge to E-mail with Attachments" at:

http://word.mvps.org/FAQs/MailMerge/MergeWithAttachments.htm

and if you go to :

http://www.gmayor.com/individual_merge_letters.htm

and you are using Word XP or later, the "Add-in to Merge Letters to Separate
Files" that I have written and that can be downloaded from that site will
allow you to create each letter as a separate file with a filename taken
from a field in the data source with a minimum of fuss.