All issue related notifications that I know of so far, include the issue key to the notification mail subject. Users receiving this notification mail can easily comment on the issue by replying on the notification mail, assuming their handler has been set up correctly.

Notification mails generated from @mentions in issues, will not include the issue key in the subject of the mail, causing it impossible to add comments by replying to the notification mail.

Could the issue key please get added for these notifications?