Spamm Azz writes:

I wrote a function in C language that customizes an email body. My target is to integrate this function inside mailman 3 to customize the email for each recipient. To manage an input email is possible inserting an handler, but I don't know how to do the same for an outgoing message.

Dunno off-hand (and my time has to be focused elsewhere for a couple weeks) but you can find what you need by looking for code and comments that mentions "personalization" or variations of that word. "Personalization" is the term used in Mailman 2 for messages that have different content for each recipient. I'm quite sure that it's used in Mailman 3, too. :-)

Steve