Incoming email

GitLab has several features based on receiving incoming emails:

Reply by Email: allow GitLab users to comment on issues and merge requests by replying to notification emails.
New issue by email: allow GitLab users to create a new issue by sending an email to a user-specific email address.
New merge request by email: allow GitLab users to create a new merge request by sending an email to a user-specific email address.

Requirements

Handling incoming emails requires an IMAP-enabled email account. GitLab requires one of the following three strategies:

Email sub-addressing
Dedicated email address
Catch-all mailbox

Let's walk through each of these options.

If your provider or server supports email sub-addressing, we recommend using it. Most features (other than reply by email) only work with sub-addressing.

Email 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, Google Apps, Yahoo! Mail, Outlook.com and iCloud, as well as the Postfix mail server which you can run on-premises.

Dedicated email address

This solution is really simple to set up: you just have to create an email address dedicated to receive your users' replies to GitLab notifications.

Catch-all mailbox

A catch-all mailbox for a domain will "catch all" the emails addressed to the domain that do not exist in the mail server.

GitLab can be set up to allow users to comment on issues and merge requests by replying to notification emails.

Set it up

If you want to use Gmail / Google Apps for incoming emails, make sure you have IMAP access enabled and allowed less secure apps to access the account or turn-on 2-step validation and use an application password.

To set up a basic Postfix mail server with IMAP access on Ubuntu, follow the Postfix setup documentation.

Security Concerns

WARNING: Be careful when choosing the domain used for receiving incoming email.

For the sake of example, suppose your top-level company domain is hooli.com. All employees in your company have an email address at that domain via Google Apps, and your company's private Slack instance requires a valid @hooli.com email address in order to sign up.

If you also host a public-facing GitLab instance at hooli.com and set your incoming email domain to hooli.com, an attacker could abuse the "Create new issue by email" or "Create new merge request by email" features by using a project's unique address as the email when signing up for Slack, which would send a confirmation email, which would create a new issue or merge request on the project owned by the attacker, allowing them to click the confirmation link and validate their account on your company's private Slack instance.

We recommend receiving incoming email on a subdomain, such as incoming.hooli.com, and ensuring that you do not employ any services that authenticate solely based on access to an email domain such as *.hooli.com. Alternatively, use a dedicated domain for GitLab email communications such as hooli-gitlab.com.

See GitLab issue #30366 for a real-world example of this exploit.

Omnibus package installations

Find the incoming_email section in /etc/gitlab/gitlab.rb, enable the feature and fill in the details for your specific IMAP server and email account:

Configuration for Postfix mail server, assumes mailbox incoming@gitlab.example.com

gitlab_rails['incoming_email_enabled'] = true

# The email address including the `%{key}` placeholder that will be replaced to reference the item being replied to.
# The placeholder can be omitted but if present, it must appear in the "user" part of the address (before the `@`).
gitlab_rails['incoming_email_address'] = "incoming+%{key}@gitlab.example.com"

# Email account username
# With third party providers, this is usually the full email address.
# With self-hosted email servers, this is usually the user part of the email address.
gitlab_rails['incoming_email_email'] = "incoming"
# Email account password
gitlab_rails['incoming_email_password'] = "[REDACTED]"

# IMAP server host
gitlab_rails['incoming_email_host'] = "gitlab.example.com"
# IMAP server port
gitlab_rails['incoming_email_port'] = 143
# Whether the IMAP server uses SSL
gitlab_rails['incoming_email_ssl'] = false
# Whether the IMAP server uses StartTLS
gitlab_rails['incoming_email_start_tls'] = false

# The mailbox where incoming mail will end up. Usually "inbox".
gitlab_rails['incoming_email_mailbox_name'] = "inbox"
# The IDLE command timeout.
gitlab_rails['incoming_email_idle_timeout'] = 60

Configuration for Gmail / Google Apps, assumes mailbox gitlab-incoming@gmail.com

gitlab_rails['incoming_email_enabled'] = true

# The email address including the `%{key}` placeholder that will be replaced to reference the item being replied to.
# The placeholder can be omitted but if present, it must appear in the "user" part of the address (before the `@`).
gitlab_rails['incoming_email_address'] = "gitlab-incoming+%{key}@gmail.com"

# Email account username
# With third party providers, this is usually the full email address.
# With self-hosted email servers, this is usually the user part of the email address.
gitlab_rails['incoming_email_email'] = "gitlab-incoming@gmail.com"
# Email account password
gitlab_rails['incoming_email_password'] = "[REDACTED]"

# IMAP server host
gitlab_rails['incoming_email_host'] = "imap.gmail.com"
# IMAP server port
gitlab_rails['incoming_email_port'] = 993
# Whether the IMAP server uses SSL
gitlab_rails['incoming_email_ssl'] = true
# Whether the IMAP server uses StartTLS
gitlab_rails['incoming_email_start_tls'] = false

# The mailbox where incoming mail will end up. Usually "inbox".
gitlab_rails['incoming_email_mailbox_name'] = "inbox"
# The IDLE command timeout.
gitlab_rails['incoming_email_idle_timeout'] = 60

Configuration for Microsoft Exchange mail server w/ IMAP enabled, assumes mailbox incoming@exchange.example.com

gitlab_rails['incoming_email_enabled'] = true

# The email address replies are sent to - Exchange does not support sub-addressing so %{key} is not used here
gitlab_rails['incoming_email_address'] = "incoming@exchange.example.com"

# Email account username
# Typically this is the userPrincipalName (UPN)
gitlab_rails['incoming_email_email'] = "incoming@ad-domain.example.com"
# Email account password
gitlab_rails['incoming_email_password'] = "[REDACTED]"

# IMAP server host
gitlab_rails['incoming_email_host'] = "exchange.example.com"
# IMAP server port
gitlab_rails['incoming_email_port'] = 993
# Whether the IMAP server uses SSL
gitlab_rails['incoming_email_ssl'] = true

Reconfigure GitLab for the changes to take effect:
```
sudo gitlab-ctl reconfigure
```
Verify that everything is configured correctly:
```
sudo gitlab-rake gitlab:incoming_email:check
```
Reply by email should now be working.

Installations from source

Go to the GitLab installation directory:
```
cd /home/git/gitlab
```

Find the incoming_email section in config/gitlab.yml, enable the feature and fill in the details for your specific IMAP server and email account:

sudo editor config/gitlab.yml

Configuration for Postfix mail server, assumes mailbox incoming@gitlab.example.com

incoming_email:
  enabled: true

  # The email address including the `%{key}` placeholder that will be replaced to reference the item being replied to.
  # The placeholder can be omitted but if present, it must appear in the "user" part of the address (before the `@`).
  address: "incoming+%{key}@gitlab.example.com"

  # Email account username
  # With third party providers, this is usually the full email address.
  # With self-hosted email servers, this is usually the user part of the email address.
  user: "incoming"
  # Email account password
  password: "[REDACTED]"

  # IMAP server host
  host: "gitlab.example.com"
  # IMAP server port
  port: 143
  # Whether the IMAP server uses SSL
  ssl: false
  # Whether the IMAP server uses StartTLS
  start_tls: false

  # The mailbox where incoming mail will end up. Usually "inbox".
  mailbox: "inbox"
  # The IDLE command timeout.
  idle_timeout: 60

Configuration for Gmail / Google Apps, assumes mailbox gitlab-incoming@gmail.com

incoming_email:
  enabled: true

  # The email address including the `%{key}` placeholder that will be replaced to reference the item being replied to.
  # The placeholder can be omitted but if present, it must appear in the "user" part of the address (before the `@`).
  address: "gitlab-incoming+%{key}@gmail.com"

  # Email account username
  # With third party providers, this is usually the full email address.
  # With self-hosted email servers, this is usually the user part of the email address.
  user: "gitlab-incoming@gmail.com"
  # Email account password
  password: "[REDACTED]"

  # IMAP server host
  host: "imap.gmail.com"
  # IMAP server port
  port: 993
  # Whether the IMAP server uses SSL
  ssl: true
  # Whether the IMAP server uses StartTLS
  start_tls: false

  # The mailbox where incoming mail will end up. Usually "inbox".
  mailbox: "inbox"
  # The IDLE command timeout.
  idle_timeout: 60

Configuration for Microsoft Exchange mail server w/ IMAP enabled, assumes mailbox incoming@exchange.example.com

incoming_email:
  enabled: true

  # The email address replies are sent to - Exchange does not support sub-addressing so %{key} is not used here
  address: "incoming@exchange.example.com"

  # Email account username
  # Typically this is the userPrincipalName (UPN)
  user: "incoming@ad-domain.example.com"
  # Email account password
  password: "[REDACTED]"

  # IMAP server host
  host: "exchange.example.com"
  # IMAP server port
  port: 993
  # Whether the IMAP server uses SSL
  ssl: true
  # Whether the IMAP server uses StartTLS
  start_tls: false

  # The mailbox where incoming mail will end up. Usually "inbox".
  mailbox: "inbox"
  # The IDLE command timeout.
  idle_timeout: 60

Enable mail_room in the init script at /etc/default/gitlab:

sudo mkdir -p /etc/default
echo 'mail_room_enabled=true' | sudo tee -a /etc/default/gitlab

Restart GitLab:
```
sudo service gitlab restart
```

Verify that everything is configured correctly:

sudo -u git -H bundle exec rake gitlab:incoming_email:check RAILS_ENV=production

Reply by email should now be working.