Actually, sending mails works fine. But the error is annoying because it causes mu4e to keep the compose buffer open. Normally, it would close the buffer and save the mail to the sent folder. So I have to do it manually every time.

The complete message I get is:

message-send-mail-with-sendmail: Sending...failed to mail for [ -oi --read-envelope-from -t ] : send was successful;

I am sure that msmtp is working perfectly since there are no errors in the log and sending the exact same mail from the terminal returns a 0 exit code.

The sendmail part of the emacs config is:

  (setq
        sendmail-program (executable-find "msmtpq")
        send-mail-function 'smtpmail-send-it
        message-sendmail-f-is-evil t
        message-sendmail-extra-arguments '("--read-envelope-from")
        message-send-mail-function 'message-send-mail-with-sendmail
        mml-secure-openpgp-sign-with-sender t
        message-kill-buffer-on-exit t
  )

Where does 'Sending...failed to' part of the message come from, and how can I debug it?