A per-user outbound gateway is a mail server, other than the Google Workspace 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@example.com might also have an email account user.name@partnerdomain.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 Google Workspace 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 user.name@partnerdomain.com. In this case, the partnerdomain.com mail server would be the user's outbound gateway.
Why you would allow or disallow using an outbound gateway
How do I allow or disallow use of an outbound gateway?
By default, users cannot use an outbound gateway. The Google Workspace mail servers deliver all outgoing mail, including mail that uses alternate From addresses.
To allow users to use an outbound gateway:
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Sign in with an administrator account to the Google Admin console.
If you aren’t using an administrator account, you can’t access the Admin console.
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Go to Menu
Apps > Google Workspace > Gmail > End User Access.
Requires having the Gmail Settings administrator privilege.
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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.
Changes can take up to 24 hours but typically happen more quickly. Learn more
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.