Add an option to allow users to specifically enable or disable
retrieving the Message-ID from the Outlook SMTP server. This can be
for other hosts mimicking the behaviour of Outlook, or for users who set
a custom domain to be a CNAME for the Outlook SMTP server.

Co-authored-by: Aditya Garg [off-list ref]
Signed-off-by: Aditya Garg <redacted>
---

diff --git a/Documentation/git-send-email.adoc b/Documentation/git-send-email.adoc

@@ -421,6 +421,21 @@ recipient's MUA.
+--[no-]smtp-outlook-id-tweak::
+       Outlook servers discard the Message-ID sent via email and assign a
+       new random Message-ID, thus breaking threads.
++
+--
+- '--smtp-outlook-id-tweak' will attempt to retrieve the ID from the server
+  irrespective of the SMTP server being used. Use only if Microsoft is your
+  email provider.
+- '--no-smtp-outlook-id-tweak' will disable this tweak irrespective of the
+  SMTP server being used.
+--
++
+If not sepcified, the default behaviour will be to enable the tweak only if the
+SMTP server is 'smtp.office365.com' or 'smtp-mail.outlook.com'.

diff --git a/git-send-email.perl b/git-send-email.perl

@@ -60,6 +60,8 @@ sub usage {
     --smtp-encryption       <str>  * tls or ssl; anything else disables.
+    --smtp-outlook-id-tweak <0|1>  * This server munges Message-ID. Retrive it from
+                                     the server and assign to \$message_id.