I thought you may take interest in some utilities I have been using.
mhq facilitates the practice of enqueuing emails in the drafts
folder and, later, sending them all at once.
https://krabben.twilightparadox.com/fossil/mhq
mhrecoll is an frontend for recoll queries. The user interface is much
like that of mairix queries. It does not index; you could use
recollindex for that. The main benefit compared to mairix is the ease of
searching files other than mail. (I have mentioned mhrecoll before.)
https://krabben.twilightparadox.com/fossil/mhrecoll
nmh-contrib contains things I haven't organized.
https://krabben.twilightparadox.com/fossil/nmh-contrib
Among the utilities in nmh-contrib are utilities for deduplicating
emails and organizing emails based on date. I am likely to organize
these into their own package some day. It is from using these utilities
that I noticed Return-Path situation that we discussed two weeks ago.
With great humility,
Krullen Van De Trap