After you have sent your e-mails, you need to wait a little while to check for returns because of errors. There are a couple of causes for returns:
Delivery Status Notification (Failure)
When an address is invalid it fails to deliver the e-mail.
Look up the address in the Contacts List to see if there is other contact information that you can use to verify the address with the Member; phone number or mailing address.
Correct the discrepancy in the Contact.
Some folks provide a Vacation notification. These can be Deleted.
Using the Google Mail System to send a Member Invitation