Add more cross-linking

Cross-link to both Exchange and 365 subheadings.

doc/administration/incoming_email.md

@@ -42,7 +42,8 @@ a mail server feature where any email to `user+arbitrary_tag@example.com` will e
 in the mailbox for `user@example.com` . It is supported by providers such as
 Gmail, Google Apps, Yahoo! Mail, Outlook.com, and iCloud, as well as the
 [Postfix mail server](reply_by_email_postfix_setup.md), which you can run on-premises.
-Microsoft Exchange Server [does not support sub-addressing](#microsoft-exchange-server).
+Microsoft Exchange Server [does not support sub-addressing](#microsoft-exchange-server),
+and Microsoft Office 365 [does not support sub-addressing by default](#microsoft-office-365)
 
 TIP: **Tip:**
 If your provider or server supports email sub-addressing, we recommend using it.