Prevent users to mention Admin accounts

In some customer environments, admin accounts are not mail enabled. If an admin account is mentioned by a user, the notification will not be delivered successfully. It's requested that chosen accounts could be set as invisible.

Steps to Reproduce

• Create an admin account and give him a fake mail address
• Create another Jira user
• Create a Jira issue and mention the account created at first step

Expected Results

When related account are tired to be mentioned, it shouldn't be found. 

Actual Results

It's allowed to mention all users in a Jira environment but if the user's mail address is not a real one, then that user will not receive any notification.

Workaround

No current workaround.