Mail Merge

Mail merge is one of those things that we would all like to do from time to time, send a personalized email to a list.  Most people are familiar with mail merge in Microsoft Word, but due to a quirk in the way Word implements Simple MAPI and a deficiency in Thunderbird, it just does not work.

The answer is simply,
either use another word processor like  LibreOffice Libre Office sends it's own mail merges without any reference to a mail program at all.  It also quite happily uses the Thunderbird address book to address the mail and is my prefered method of mail merging..

Or

Install the Mail Merge add-on for Thunderbird and do it all from Thunderbird,