I don't know offhand of any good mail merge programs for the mac.
While "kevs" may not need it, for general future reference check out
Serial Mailer. It allows for smart field inserts that can change with each duplicate e-mail, customized to a particular recipient.
And I second the suggestion of cut and paste and save 10 copies in the Drafts folder of Apple Mail. Each draft is visually and physically distinct from all the others.
One last comment: I have seen sent messages still saved in Drafts, even though they have been sent. It was not a consistent issue in the past and does not seem to have popped up with the latest version of OS 10.10.
