Smtpmail: Automatically set smtpmail-smtp-user.

.emacs.d/lisp/init-smtpmail.el

@@ -5,12 +5,20 @@
       smtpmail-stream-type 'starttls
       smtpmail-smtp-service 587)
 
-;; This is only useful to distinguish between similar entries in .authinfo.
-;; (setq smtpmail-smtp-user user-mail-address)
-
 ;; REVIEW: If we don't set `user-mail-address', `mail-host-address' or
 ;; `message-user-fqdn', `message-make-fqdn' will put
 ;; "i-did-not-set--mail-host-address--so-tickle-me" in the In-Reply-To header.
 (setq user-mail-address "mail@ambrevar.xyz")
 
+;; This is only useful to distinguish between similar entries in .authinfo / password-store.
+(defun ambrevar/set-smtp-user ()
+  "Set `smtpmail-smtp-user' to the value in the \"From\" field."
+  (let ((header-value (message-fetch-field "From")))
+    (and
+     header-value
+     (setq smtpmail-smtp-user
+           (substring-no-properties
+            (cadr (mail-extract-address-components header-value)))))))
+(add-hook 'message-send-hook #'ambrevar/set-smtp-user)
+
 (provide 'init-smtpmail)