Outbound relay

An outbound relay is a mail server other than the Google Apps mail servers that delivers outgoing mail for a user in your domain.

Gmail offers users the option to associate additional email addresses with their accounts, to create custom From addresses. (Learn more about how the user does it.) For example, user@yourdomain.com might also have an email account user.name@partnerdomain.com. When the user sends or responds to a message from Gmail, he or she wants to be able to choose which email address appears in the From field.

By default, the Google Apps mail servers deliver the user's outgoing messages regardless of which email address he or she chooses to send from. However, the user might want the mail server for partnerdomain.com to deliver any message that uses the address user.name@partnerdomain.com. In this situation, the user wants to use the partnerdomain.com mail server as an outbound relay.

What is the difference between an outbound relay and an outbound gateway?

An outbound gateway is a mail server through which all of your domain's outgoing mail flows. An outbound relay is a mail server used to send messages from email addresses that are not part of your domain (despite being sent by a user who also has an account in your domain). Because the messages handled by an outbound relay do not come from your domain, they do not flow through your domain's outbound gateway. (Learn more about outbound gateways.)

Why would a user use an outbound relay?

Using an outbound relay ensures that the same mail server delivers all messages from user.name@partnerdomain.com, regardless of whether he or she composes the message using Gmail or another email client application. Recipients can verify that the message is valid by confirming that it comes from the proper mail server for partnerdomain.com. Also, the partnerdomain.com mail server has a record of the mail sent from its domain using Gmail. At the same time, the mail sent from the partnerdomain.com is not handled by the Google Apps mail servers or your domain's outbound gateway, which may be what you want.

Using an outbound relay can also prevent the appearance of "on behalf of" addresses in the From field. When a user sends a message from an alternate From address through the Google Apps mail servers, some recipient mail applications display the name of the sender as "user.name@partnerdomain.com on behalf of user@yourdomain.com".

Learn more about how the user configures an outbound relay. The user has the option only if you allow the option from the Google Apps Control Panel, as described below.

Why not allow users to use an outbound relay?

Messages delivered using an outbound relay do not flow through any outbound gateway specified for your domain. If you need to ensure that all outgoing mail from your users passes through the outbound gateway, disable users' option to specify an outbound relay.

How do I allow or disallow users to use an outbound relay?

By default, users cannot use an outbound relay; the Google Apps mail servers deliver all outgoing mail, including mail that uses alternate From addresses.

To allow users to use an outbound relay:

  1. Log into the Google Apps control panel for your domain.
  2. Click the Settings tab and then select Email in the left column.
  3. In the Outbound relay section, select Allow users to send mail through an external SMTP when configuring a "from" address hosted outside your domain.

    When this check box is selected, users who associate additional email addresses with their accounts have the option to enter connection information for an alternate SMTP server. (Learn more about the required steps for the user.) When the check box is not selected, users cannot specify an SMTP server for the additional address.
  4. Click Save changes.

It can take up to an hour for the changes to propagate through Google systems.

Once this option is enabled, users can configure their alternate email addresses to use an outbound relay. Each user must configure this option individually. Learn more about how the user does it.