Mail Connectors → Mail Senders → (name) → Advanced

The Domain Filtering box lets you restrict which email messages are sent via this Mail Sender.

You specify each rule on a separate line, and the message will be sent if it matches any sender rule and any recipient rule. If there are only sender rules or only recipient rules, then messages will be sent if there it matches any of that type of rule.

You can specify either the sender as
From: <wildcard>
e.g. From: *@mydomain.com or From: joe@mydomain.com

or the recipient as
<wildcard domain>
e.g. customerdomain.com or customerdomain.*

So a sample might be
From: joe@mydomain.com
From: sales@mydomain.com
customerdomain.com
*microsoft.com

This would use this Mail Sender to send messages from either joe@mydomain.com or sales@mydomain.com AND to either someone@customerdomain.com or someone@<something>microsoft.com.