Hi, we recently ran into this problem.
A user has edited his profile details, by changing "email address" field.
He entered TWO email addres separated by semicolon (e.g. "foo@bar.com; bar@foo.com").
From that moment all the notification emails to his profile where blocked in Mail Error Queue (because of the invalid email address).
I know that Jira doesn't support multiple email addresses per user, but it would be nice to tell to the user that the email addres he entered is invalid.

Thanks,
Davide.