Send email when manually mentioning a user with [~username]?

[Andreas P]

As external users should not be allowed to see all users (e.g. in the "Add watchers" field), they don't have the global permission "Browse users". Therefore they can't mention users with @username either. As they still need to do this, I thought manually mentioning the users with [~username] would work. And the username inside the brackets is actually converted into a link to the mentioned user, but the mentioned user doesn't get informed via email.

Is there a way to configure this, so that the mentioned user (let's call him John) receives his notification email?

(If a not-external user mentions John, John receives a notification email. John has access to the issue he is mentioned in. So that's not the problem.) 

Or is there another way those external users can mention John in a way he also receives the email?