One way would be to use a DATABASE field in a normal ‘letter’ mailmerge main document and a macro to drive the process. An outline of this approach can be found at:
https://answers.microsoft.com/en-us/...1-1996c14dca5d
Conversely, if you're using a relational database or, Excel workbook with a separate table with just a single instance of each of the grouping criteria, a DATABASE field in a normal ‘letter’ mailmerge main document could be used without the need for a macro. An outline of this approach can be found at:
https://answers.microsoft.com/en-us/...f-8642e46fa103
For some working examples (though not necessarily for email), see:


https://www.msofficeforums.com/mail-...-multiple.html
Merge excel list into Word Receipt

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]