One general solution to this quandary is to input a dummy email address for people who have obviously entered an incorrect addresses or who don't have an email account. That's one solution.
But this doesn't prevent emails from being sent. Today let's work on improving our previous workflows introduced in "Creating a Workflow for Managing Requests for Samples" and "Responding Quickly to Communications in Sample Requests."
[Sample Request <No Email>: "Thank You Email" settings screen]
Now for the improvement: in the below workflow, if the email address begins with "info" and ends with "@questetra.com," an email won't be sent. In other words, emails that look like "info+2011@questetra.com" will not receive a thank you note.