This allows for extra authentication mechanisms (such as XOAuth2) to
be added without having to advise any functions.

* lisp/mail/smtpmail.el: Turn `smtpmail-try-auth-method' into a
generic function.
---
 lisp/mail/smtpmail.el | 33 +++++++++++++++++++++++++++++++--
 1 file changed, 31 insertions(+), 2 deletions(-)

diff --git a/lisp/mail/smtpmail.el b/lisp/mail/smtpmail.el
index 20cbeb5f4e..46c1639832 100644
--- a/lisp/mail/smtpmail.el
+++ b/lisp/mail/smtpmail.el
@@ -181,7 +181,25 @@ This is relative to `smtpmail-queue-dir'."
 
 (defconst smtpmail-auth-supported '(cram-md5 plain login)
   "List of supported SMTP AUTH mechanisms.
-The list is in preference order.")
+The list is in preference order.
+
+New mechanisms can be added by defining a method implementing the
+`smtpmail-try-auth-method' generic function for that mechanism, and
+then adding the mechanism to this list.  As an example, the code below
+adds support for XOAuth2 authentication (assuming that there is an
+appropriate auth source extension that can provide the correct user
+and password):
+
+\(cl-defmethod smtpmail-try-auth-method
+    (process (mech (eql xoauth2)) user password)
+  (smtpmail-command-or-throw
+   process
+   (concat \"AUTH XOAUTH2 \"
+           (base64-encode-string
+            (concat \"user=\" user \"\\1auth=Bearer \" password \"\\1\\1\") t))
+   235))
+
+\(add-to-list 'smtpmail-try-auth-method 'xoauth2)")
 
 (defvar smtpmail-mail-address nil
   "Value to use for envelope-from address for mail from ambient buffer.")
@@ -539,7 +557,18 @@ The list is in preference order.")
 	  (funcall save-function))
 	result))))
 
-(defun smtpmail-try-auth-method (process mech user password)
+(cl-defgeneric smtpmail-try-auth-method (process mech user password)
+  "Try to authenticate to an SMTP process.
+Authenticate to PROCESS using the MECH authentication mechanism, with
+the given USER and PASSWORD. Implementations should either return the
+result code from the authentication command, or throw 'done with an
+error string. (This is usually done by calling
+`smtpmail-command-or-throw' as the last step in the function.)")
+
+(cl-defmethod smtpmail-try-auth-method (process mech user password)
+  "Default `smtpmail-try-auth-method' implementation.
+This implementation provides support for the SMTP authentication
+mechanisms 'cram-md5, 'login, and 'plain."
   (let (ret)
     (cond
      ((or (not mech)
-- 
2.15.0

-- 
Cesar Crusius