Email Notification in Test System

We have a sandbox (test) system for a customer and uploaded few user accounts via workbook.

Those users with the sap domain name in the email address got the notfication email about new account but those users with non-sap domain name in the email address didn't get the email notification about new account. Checking the Processed messages I see that these notfications had failed.

Is external email notification disabled in test system?

Thanks,

Manoj