In order to process incoming e-mail messages as issues registered in GitLab you need to have catch-all mailbox and GitLab configured to handle it. However, there are few quirks you need to know when setting it up.

Assuming you have GitLab installed as well as iRedMail.

iRedMail

First, create new mailbox in iRedMail admin panel. Then manually create forwarding record. You could skip this is you purchased iRedMail Pro:

INSERT INTO forwardings 
  (address, forwarding, domain, dest_domain)
VALUES 
  ('domain.com', 'catchall@domain.com', 'domain.com', 'domain.com');

At this point you can check if catch-all feature is working by sending mail to non existing mailbox user. What is especially important is to verify if messages will come once sent to catchall+something@domain.com address. At first they will appear as separate mailbox folders, which is no good from GitLab perspective. However it is easy to fix with removing extensions parameter from dovecot. So, change this:

dovecot unix    -       n       n       -       -      pipe
    flags=DRh ... -m ${extension}

to this one:

dovecot unix    -       n       n       -       -      pipe
    flags=DRh ...

And run /etc/init.d/postix restart

GitLab

As for GitLab itself it is straightforward. Just edit /etc/gitlab/gitlab.rb at incoming_email* parameters. You need to use IMAP however so choose between 143 ad 993 ports. First one is intended to use STARTTLS and second one is for TLS. In case of a default iRedMail installation there will be self-signed certificate and incoming email will not work which you can by the way with the following command:

sudo gitlab-rake gitlab:incoming_email:check

Once you fill in all the required parameters:

sudo gitlab-ctl reconfigure

Remember that all configuration you have changed in PostgreSQL will be discarded (/var/opt/gitlab/postgresql/data/pg_hba.conf and /var/opt/gitlab/postgresql/data/postgresql.conf) so need to take care of it either by templating it or placing in GitLab main configuration.

Get iRedMail certificate. You can grab it using browser. Save it as .crt file in /etc/gitlab/trusted-certs directory. After that reconfigure and check connectivity once again. You should be good to go.

How to operate?

In GitLab a service desk is enabled in all the projects created, automatically. You can find it Monitor – Service Desk. You will find there also a target email address to which you will be sending your service desk messages. Once mail is sent it will appear in this section as an issue. You can comment and reply thru email client directly without need to visit GitLab itself.