Seb <spluque@gmail.com> writes:

> I'm using msmtp to send mail through SMTP.  I have two SMTP servers
> defined in ~/.msmtp, and setting the following:
>
>       message-send-mail-function 'message-send-mail-with-sendmail
>       sendmail-program "/usr/bin/msmtp"
>       message-sendmail-extra-arguments '("-a" "gmail")
>
> does send mail through the "gmail" account in ~/.msmtp.  Now I need to
> send mail depending on the group.

I assume you use different sender address too and you set it with
gnus-posting-styles. If this is your case then I can recommend adding
the following to your global configuration

(setq message-sendmail-f-is-evil nil
      message-sendmail-envelope-from 'header)

From now on msmtp will be started with '-f <from-addr>' will choose the
appropriate account.

-- 
Miłego dnia,
Łukasz Stelmach