Sheetmailer: https://sheetmailer.io/

It allows users to create personalized emails from the data in an Excel sheet. They write a template like

   Hello {Name}
   Your grade of todays exam was {Grade}.
   Best regards
   Prof. X

The mailer automatically fills out the gaps with the corresponding column of the Excel sheet.

Since it's been running, I got a single one-time payment (about four years ago). I'm using it a lot myself though.

Luckily, it's almost free to run (on a 13$/m Hetzner machine, shared with about 10 other applications). And as a teacher, I used it to send over 4000 emails containing grades and similar stuff to my students.