diff --git a/.emacs.d/lisp/init-smtpmail.el b/.emacs.d/lisp/init-smtpmail.el
index df3893c6..6162e873 100644
--- a/.emacs.d/lisp/init-smtpmail.el
+++ b/.emacs.d/lisp/init-smtpmail.el
@@ -5,6 +5,12 @@
       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")
+
 (provide 'init-smtpmail)