This looping is done for you.
If you have a secondary data source with the Sharepoint list.
Add a rule to the email entry field. Make the condition emailentryfield = email in SP list. Then have the action be whatever you want, message box or whatever.
When InfoPath does the check it actually checks each value in the repeating nodes and if any or more match it will return true, if no matches false.
You should be able to test this and gain more understanding.