This will happen if someone enters an invalid e-mail address when subscribing.  As an example if they enter 'myname@mydomain' (without the .com).  Another common mistake is people will try to enter two e-mail addresses in the subscription field when they should really just subscribe each address separately.

 

Go to Members > Admin > Subscriptions and look at the subscriptions in the list.  If you see an e-mail address entry in the list that is invalid you can enter it correctly and remove the one that is incorrect.