We are working on an normal SharePoint(20101) list form and when domain users submit any item on the list with external user name(outside domain), an auto email need to send external user saying as item has been submitted in form with number "xxxxxxx". Means email notification need to go for external users based on "External user column" name. is it available in SharePoint 20101? sending email to external domain users?
You can create a SharePoint Designer workflow to achieve it. And in this workflow, use "Person is a valid SharePoint user" condition and insert a "Send an Email" action into the condition.
More information, you can refer to: Workflow Condition: person is a valid sharepoint user with Example