I’m a longtime fan and user of Tinderbox, but it wasn’t until recently that I realized it could be the solution to my canned email problem.
Each template message is its own Tinderbox note. The body of the note contains boilerplate text with value substitution calls that insert data from the note’s User Attributes. The example screenshot attached should clarify.
When I need to send an email, I duplicate the template note, alter the field data, then export the note to plain text. Then I delete the duplicate, just to keep things tidy. The contents of the plain text export are copied to an email message. Done and done.