Email addresses for people that are of an invalid format cause errors when the system attempts to send an message to that email address. For instance, a person with an email of "n/a" errored out when attempting to close the ticket. The email sending code was crashing because it presumes at least a well formed email address.
The backend relies on a properly formatted email, if an email.
On the backend #377:
This issue patches this up on the frontend.