It is possible to see the E-mail delivery status under the "User Options" menu located in the Sharing menu. There are 3 colors to indicate the status.
Green: All OK E-mail is delivered
Orange: The is E-mail is awaiting delivery
Red: Fail delivery; most often, this is a typing error in the E-mail address.
If you click on the E-mail, you can open the user options for each user and see the status information for the E-mail delivery. If there is an error and you are not able to understand it, then just mail to firstname.lastname@example.org