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

[rameiko87]

Dear Eli,
Thanks very much for your swift reply. Please understand that I mean no 
disturbance through my questions, I'm only trying to understand. What is 
confusing me still is that mail-user-agent sets the composition package 
for outgoing emails (in my personal configuration this is the Message 
package, in my mind this is the MUA being used when writing emails). The 
message-mail-user-agent variable instead it's not clear what is setting.

> "Your preferred mail composition package when reading email with 
> message.el.

But I read my emails with Rmail. Perhaps you meant "when composing 
emails with message.el"? If yes, I thought that the message package was 
the email composition package and that I assigned this with the variable 
mail-user-agent, so I'm not sure what I'm setting here.

>   Like `mail-user-agent' (which see), this specifies the mail-sending 
> package you prefer.

Now the composition package from the first quote turned into sending 
package! What is a mail sending package? How is this variable the 
"analogous" of mail-user-agent (I really don't like that "Like")?

>   The value can be any value accepted by `mail-user-agent', and in 
> addition it can be nil or t.  If the value is nil, use the Gnus native 
> Mail User Agent (MUA); if it is t, use the value of `mail-user-agent'."

Why is the default the Gnus package? The problem is that I still don't 
understand the difference with mail-user-agent.