Subject: bug#67638: Description of the variable message-mail-user-agent in Emacs 29.1

From: Eli Zaretskii <[email-masked]>
Date: Thu, 07 Dec 2023 19:14:11 +0200

> Date: Thu, 07 Dec 2023 14:47:20 +0000
> From: [email-masked]
> Cc: [email-masked]
> 
> Can you please kindly explain in one sentence where 
> message-mail-user-agent and mail-user-agent intervene during the process 
> of replying to an email message which I'm reading using Rmail?

When you type 'r', Emacs invokes rmail-reply, which calls
rmail-start-mail, which calls compose-mail.  compose-mail invokes the
composition function of mail-user-agent, and when you invoke the
command to send the response, Emacs invokes the send-mail function of
mail-user-agent.

rmail-start-mail (and thus compose-mail) is also called by commands
rmail-mail, rmail-continue, rmail-forward, and a couple of others.