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2 files changed, 321 insertions, 3 deletions

diff --git a/doc/reply_by_email/README.md b/doc/reply_by_email/README.md
index 67dc2b04000..accd83b66df 100644
--- a/doc/reply_by_email/README.md
+++ b/doc/reply_by_email/README.md
@@ -2,11 +2,13 @@
 
 GitLab can be set up to allow users to comment on issues and merge requests by replying to notification emails.
 
-In order to do this, you need access to an IMAP-enabled email account, with a provider or server that supports [email sub-addressing](https://en.wikipedia.org/wiki/Email_address#Sub-addressing). Sub-addressing is a feature where any email to `user+some_arbitrary_tag@example.com` will end up in the mailbox for `user@example.com`, and is supported by providers such as Gmail, Yahoo! Mail, Outlook.com and iCloud, as well as the [Postfix](http://www.postfix.org/) mail server which you can run on-premises.
+## Get a mailbox
 
-If you want to use a Gmail with Reply by email, make sure you have [IMAP access enabled](https://support.google.com/mail/troubleshooter/1668960?hl=en#ts=1665018) and [allow less secure apps to access the account](https://support.google.com/accounts/answer/6010255).
+Reply by email requires an IMAP-enabled email account, with a provider or server that supports [email sub-addressing](https://en.wikipedia.org/wiki/Email_address#Sub-addressing). Sub-addressing is a feature where any email to `user+some_arbitrary_tag@example.com` will end up in the mailbox for `user@example.com`, and is supported by providers such as Gmail, Yahoo! Mail, Outlook.com and iCloud, as well as the Postfix mail server which you can run on-premises.
 
-To set up a basic Postfix mail server with IMAP access on Ubuntu, follow [these instructions](https://help.ubuntu.com/community/PostfixBasicSetupHowto).
+If you want to use Gmail with Reply by email, make sure you have [IMAP access enabled](https://support.google.com/mail/troubleshooter/1668960?hl=en#ts=1665018) and [allow less secure apps to access the account](https://support.google.com/accounts/answer/6010255).
+
+To set up a basic Postfix mail server with IMAP access on Ubuntu, follow [these instructions](./postfix.md).
 
 ## Set it up
 
diff --git a/doc/reply_by_email/postfix.md b/doc/reply_by_email/postfix.md
new file mode 100644
index 00000000000..bab107b958d
--- /dev/null
+++ b/doc/reply_by_email/postfix.md
@@ -0,0 +1,316 @@
+# Set up Postfix for Reply by email
+
+This document will take you through the steps of setting up a basic Postfix mail server with IMAP authentication on Ubuntu, to be used with Reply by email.
+
+The instructions make the assumption that you will be using the email address `replies@gitlab.example.com`, that is, username `replies` on host `gitlab.example.com`. Don't forget to change it to your actual host when executing the example code snippets.
+
+## Configure your server DNS
+
+1. Add an MX record pointing from `gitlab.example.com` to your server IP.
+1. Add an A record pointing from `mail.gitlab.example.com` to your server IP.
+1. Verify that the changes have propagated:
+
+    ```sh
+    dig mx gitlab.example.com
+    ```
+
+    ```sh
+    dig a mail.gitlab.example.com
+    ```
+
+    You should see an `ANSWER SECTION` with the expected result in the output for both.
+
+## Install packages
+
+1. Install the `postfix` package if it is not installed already:
+
+    ```sh
+    sudo apt-get install postfix
+    ```
+
+    When asked about the environment, select 'Internet Site'. When asked to confirm the hostname, make sure it matches `gitlab.example.com`.
+
+1. Install the `mailutils` package.
+
+    ```sh
+    sudo apt-get install mailutils
+    ```
+
+## Create user
+
+1. Create a user for replies.
+
+    ```sh
+    sudo useradd -m -s /bin/bash replies
+    ```
+
+1. Set a password for this user.
+
+    ```sh
+    sudo passwd replies
+    ```
+
+    Be sure not to forget this, you'll need it later.
+
+## Test the out-of-the-box setup
+
+1. Connect to the local SMTP server:
+    
+    ```sh
+    telnet localhost 25
+    ```
+
+    You should see a prompt like this:
+
+    ```sh
+    Trying 127.0.0.1...
+    Connected to localhost.
+    Escape character is '^]'.
+    220 gitlab.example.com ESMTP Postfix (Ubuntu)
+    ```
+
+    If you get a `Connection refused` error instead, check if `postfix` is running:
+
+    ```sh
+    sudo postfix status
+    ```
+
+    If it is not, start it:
+
+    ```sh
+    sudo postfix start
+    ```
+
+1. Send the new `replies` user a dummy email to test SMTP, by entering the following into the SMTP prompt:
+    
+    ```
+    ehlo localhost
+    mail from: root@localhost
+    rcpt to: replies@localhost
+    data
+    Subject: Re: Some issue
+
+    Sounds good!
+    .
+    quit
+    ```
+
+    (Note: The `.` is a literal period on its own line)
+
+1. Check if the `replies` user received the email:
+    
+    ```sh
+    sh - replies
+    mail
+    ```
+
+    You should see output like this:
+
+    ```
+    "/var/mail/replies": 1 message 1 unread
+    >U   1 root@localhost                           59/2842  Re: Some issue
+    ```
+
+    Quit the mail app:
+
+    ```sh
+    q
+    ```
+
+1. Log out of the `replies` account and go back to being `root`:
+
+    ```sh
+    logout
+    ```
+
+## Configure Postfix to use Maildir-style mailboxes
+
+Courier, which we will install later to add IMAP authentication, requiers mailboxes to have the Maildir format, rather than mbox.
+
+1. Configure Postfix to use Maildir-style mailboxes:
+    
+    ```sh
+    sudo postconf -e "home_mailbox = Maildir/"
+    sudo postconf -e "mailbox_command = "
+    ```
+
+1. Restart Postfix:
+    
+    ```sh
+    sudo postfix restart
+    ```
+
+1. Test the new setup:
+    
+    1. Follow steps 1 and 2 of "Test the out-of-the-box setup".
+    2. Check if the `replies` user received the email:
+    
+        ```sh
+        sh - replies
+        MAIL=/home/replies/Maildir
+        mail
+        ```
+
+        You should see output like this:
+
+        ```
+        "/home/replies/Maildir": 1 message 1 unread
+        >U   1 root@localhost                           59/2842  Re: Some issue
+        ```
+
+        Quit the mail app:
+
+        ```sh
+        q
+        ```
+
+1. Log out of the `replies` account and go back to being `root`:
+
+    ```sh
+    logout
+    ```
+
+## Install the Courier IMAP server
+    
+1. Install the `courier-imap` package:
+
+    ```sh
+    sudo apt-get install courier-imap
+    ```
+
+## Configure Postfix to receive email from the internet
+
+1. Let Postfix know about the domains that it should consider local:
+    
+    ```sh
+    sudo postconf -e "mydestination = mail.gitlab.example.com, localhost.localdomain, localhost, gitlab.example.com"
+    ```
+
+1. Let Postfix know about the IPs that it should consider part of the LAN:
+    
+    We'll assume `192.168.1.0/24` is your local LAN. You can safely skip this step if you don't have other machines in the same local network.
+    
+    ```sh
+    sudo postconf -e "mynetworks = 127.0.0.0/8, 192.168.1.0/24"
+    ```
+
+1. Configure Postfix to receive mail on all interfaces, which includes the internet:
+    
+    ```sh
+    sudo postconf -e "init_interfaces = all"
+    ```
+
+1. Configure Postfix to receive mail on both IPv4 and IPv6 protocols:
+    
+    ```sh
+    sudo postconf -e "inet_protocols = all"
+    ```
+
+## Test the final setup
+
+1. Test SMTP under the new setup:
+    
+    1. Connect to the SMTP server:
+        
+        ```sh
+        telnet mail.gitlab.example.com 25
+        ```
+
+        You should see a prompt like this:
+
+        ```sh
+        Trying 123.123.123.123...
+        Connected to mail.gitlab.example.com.
+        Escape character is '^]'.
+        220 gitlab.example.com ESMTP Postfix (Ubuntu)
+        ```
+
+        If you get a `Connection refused` error instead, make sure your firewall is setup to allow inbound traffic on port 25.
+
+    1. Send the `replies` user a dummy email to test SMTP, by entering the following into the SMTP prompt:
+        
+        ```
+        ehlo gitlab.example.com
+        mail from: root@gitlab.example.com
+        rcpt to: replies@gitlab.example.com
+        data
+        Subject: Re: Some issue
+
+        Sounds good!
+        .
+        quit
+        ```
+
+        (Note: The `.` is a literal period on its own line)
+
+    1. Check if the `replies` user received the email:
+    
+        ```sh
+        sh - replies
+        MAIL=/home/replies/Maildir
+        mail
+        ```
+
+        You should see output like this:
+
+        ```
+        "/home/replies/Maildir": 1 message 1 unread
+        >U   1 root@gitlab.example.com                           59/2842  Re: Some issue
+        ```
+
+        Quit the mail app:
+
+        ```sh
+        q
+        ```
+
+    1. Log out of the `replies` account and go back to being `root`:
+
+        ```sh
+        logout
+        ```
+
+1. Test IMAP under the new setup:
+    
+    1. Connect to the IMAP server:
+        
+        ```sh
+        telnet mail.gitlab.example.com 143
+        ```
+
+        You should see a prompt like this:
+
+        ```sh
+        Trying 123.123.123.123...
+        Connected to mail.example.gitlab.com.
+        Escape character is '^]'.
+        - OK [CAPABILITY IMAP4rev1 UIDPLUS CHILDREN NAMESPACE THREAD=ORDEREDSUBJECT THREAD=REFERENCES SORT QUOTA IDLE ACL ACL2=UNION] Courier-IMAP ready. Copyright 1998-2011 Double Precision, Inc.  See COPYING for distribution information.
+        ```
+
+    1. Sign in as the `replies` user to test IMAP, by entering the following into the IMAP prompt:
+
+        ```
+        a login replies PASSWORD
+        ```
+
+        Replace PASSWORD by the password you set on the `replies` user earlier.
+
+        You should see output like this:
+
+        ```
+        a OK LOGIN Ok.
+        ```
+
+    1. Disconnect from the IMAP server:
+
+        ```sh
+        a logout
+        ```
+
+## Done!
+
+If all the tests went all right, Postfix is all set up and ready to receive email! Continue with the [Reply by email](./README.md) doc to configure GitLab.
+
+---------
+
+_This document was adapted from https://help.ubuntu.com/community/PostfixBasicSetupHowto, by contributors to the Ubuntu documentation wiki._