This hook can be used to update the message based on the results of
address completion. For example I use it to set the From address based
on the To address just completed.

The post-completion command is added to the notmuch-company backend to
ensure that the hook is also called company completion is started
without going through notmuch-address-expand-name. See the docstring of
`company-backends' for more information.
---
 emacs/notmuch-address.el | 10 ++++++++++
 emacs/notmuch-company.el |  1 +
 2 files changed, 11 insertions(+)

diff --git a/emacs/notmuch-address.el b/emacs/notmuch-address.el
index b2e1fba..17960bf 100644
--- a/emacs/notmuch-address.el
+++ b/emacs/notmuch-address.el
@@ -98,6 +98,12 @@ to know how address selection is made by default."
   :group 'notmuch-send
   :group 'notmuch-external)
 
+(defcustom notmuch-address-completion-hook nil
+  "Functions called after completing address. The completed address is passed as an argument to each"
+  :type 'hook
+  :group 'notmuch-address
+  :group 'notmuch-hooks)
+
 (defun notmuch-address-selection-function (prompt collection initial-input)
   "Call (`completing-read'
       PROMPT COLLECTION nil nil INITIAL-INPUT 'notmuch-address-history)"
@@ -170,6 +176,10 @@ external commands."
     (process-lines notmuch-address-command original))))
 
 (defun notmuch-address-expand-name ()
+  (let ((return-value (notmuch-address-real-expand-name)))
+    (run-hook-with-args 'notmuch-address-completion-hook)))
+
+(defun notmuch-address-real-expand-name ()
   (cond
    ((and (eq notmuch-address-command 'internal)
 	 notmuch-address-use-company
diff --git a/emacs/notmuch-company.el b/emacs/notmuch-company.el
index 168315f..91c4804 100644
--- a/emacs/notmuch-company.el
+++ b/emacs/notmuch-company.el
@@ -86,6 +86,7 @@
       (match (if (string-match notmuch-company-last-prefix arg)
 		 (match-end 0)
 	       0))
+      (post-completion (run-hook-with-args 'notmuch-address-completion-hook arg))
       (no-cache t))))
 
 
-- 
2.10.1