Merge branch 'update_mail_docs' into 'master'

Update mailroom/postfix documentation [ci skip]

I worked with a customer to troubleshoot some strange Postfix and Mailroom issues. These doc updates aim to help other users avoid the same confusion.


See merge request !2430

doc/incoming_email/README.md

@@ -74,10 +74,11 @@ To set up a basic Postfix mail server with IMAP access on Ubuntu, follow [these
 
     As mentioned, the part after `+` in the address is ignored, and any email sent here will end up in the mailbox for `incoming@gitlab.example.com`/`gitlab-incoming@gmail.com`.
 
-1. Reconfigure GitLab for the changes to take effect:
+1. Reconfigure GitLab and restart mailroom for the changes to take effect:
 
     ```sh
     sudo gitlab-ctl reconfigure
+    sudo gitlab-ctl restart mailroom
     ```
 
 1. Verify that everything is configured correctly:

doc/incoming_email/postfix.md

@@ -84,7 +84,12 @@ The instructions make the assumption that you will be using the email address `i
     quit
     ```
 
-    (Note: The `.` is a literal period on its own line)
+    _**Note:** The `.` is a literal period on its own line._
+
+    _**Note:** If you receive an error after entering `rcpt to: incoming@localhost`
+    then your Postfix `my_network` configuration is not correct. The error will
+    say 'Temporary lookup failure'. See
+    [Configure Postfix to receive email from the Internet](#configure-postfix-to-receive-email-from-the-internet)._
 
 1. Check if the `incoming` user received the email:
     
@@ -131,7 +136,7 @@ Courier, which we will install later to add IMAP authentication, requires mailbo
 1. Test the new setup:
     
     1. Follow steps 1 and 2 of _[Test the out-of-the-box setup](#test-the-out-of-the-box-setup)_.
-    2. Check if the `incoming` user received the email:
+    1. Check if the `incoming` user received the email:
     
         ```sh
         su - incoming
@@ -152,6 +157,12 @@ Courier, which we will install later to add IMAP authentication, requires mailbo
         q
         ```
 
+    _**Note:** If `mail` returns an error `Maildir: Is a directory` then your
+    version of `mail` doesn't support Maildir style mailboxes. Install
+    `heirloom-mailx` by running `sudo apt-get install heirloom-mailx`. Then,
+    try the above steps again, substituting `heirloom-mailx` for the `mail`
+    command._
+
 1. Log out of the `incoming` account and go back to being `root`:
 
     ```sh