Another option would be to use a DATABASE field in a normal ‘email’ mailmerge main document and a macro to drive the process. An outline of this approach can be found at:
Many to one email merge using tables - Microsoft Community
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 ‘email’ mailmerge main document could be used without the need for a macro. An outline of this approach can be found at:
Mail Merge - To a Word Table on a Single Page - Microsoft Community
For some working examples (though not specifically for emails), see:
Mail Merge: Using One Excel File with Multiple Sheets
Merge excel list into Word Receipt