A per-user outbound gateway is a mail server, other than the G Suite 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, firstname.lastname@example.org might also have an email account email@example.com. When the user sends or responds to a message from Gmail, they want to be able to choose which email address appears in the From field.
By default, the G Suite mail servers deliver the user's outgoing messages regardless of which email address they choose to send from. However, the user might want the partnerdomain.com mail server to deliver messages using the address firstname.lastname@example.org. In this case, the partnerdomain.com mail server would be the user's outbound gateway.
Why you would allow or disallow using an outbound gatewayWhy you'd allow use of an outbound gateway
Using an outbound gateway ensures that the same mail server delivers all messages from email@example.com, regardless of whether they compose 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 G Suite mail servers or your domain's outbound gateway, which can be what you want.
Using an outbound gateway 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 G Suite mail servers, some recipient mail applications display the name of the sender as "firstname.lastname@example.org on behalf of email@example.com".
Learn more about how the user configures an outbound gateway. The user has the option only if you allow the option from the Google Admin console, as described below.
Messages delivered using an outbound gateway 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 gateway.
How do I allow or disallow use of an outbound gateway?
By default, users cannot use an outbound gateway. The G Suite mail servers deliver all outgoing mail, including mail that uses alternate From addresses.
To allow users to use an outbound gateway:
From the Admin console Home page, go to AppsG SuiteGmailEnd User Access.
Tip: To see End User Access, scroll to the bottom of the Gmail page.
In the Allow per-user outbound gateways section, select Allow users to send mail through an external SMTP server when configuring a "from" address hosted outside your email domains.
When this check box is selected, users who associate additional email addresses with their accounts can 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.
At the bottom, click Save.
It can take up to 24 hours for changes to propagate to all user accounts.
Once this option is enabled, users can configure their alternate email addresses to use an outbound gateway. Each user must configure this option individually. Learn more about how the user does it.