I am so close to getting my ideal email setup working...

Instead of offlineimap or mbsync, I use gmailieer to pull down emails. It
is quite fast and works with notmuch to map Gmail labels to notmuch tags.
Much cleaner than the awkward way of accessing Gmail through IMAP. Where I
am stuck is in sending the email.

I could set up smtpmail as described in
https://notmuchmail.org/emacstips/#index12h2 but gmailieer recently (v1.1)
gained the capability to send mail. I would like to use that if I can so.

Gmailieer sends a draft through Gmail without a problem (once I create a
draft using message mode in emacs). Both sending a file and sending from
stdin work:
$ gmi send -C path/to/mailbox 1586809748.A123456Z12345.test\:2\,D
$ cat 1586809748.A123456Z12345.test\:2\,D | gmi send -C path/to/mailbox

But I am stuck configuring emacs to use gmailieer instead of sendmail or
smtpmail.el. According to the emacs manual, the variable send-mail-function
can be set but none of the choices looks appropriate for gmailieer:
https://www.gnu.org/software/emacs/manual/html_node/emacs/Mail-Sending.html

Anyone have an idea how to configure emacs to use gmailieer to send emails?

mkg