RT sends email to disabled users

I am a bit surprised, that our RT instance sends emails to disabled
users. I thought disabling user is sufficient procedure tu suppress
user access and all communication towards that user by RT.

I did some RT database cleanup procedures (adding/deleting watchers on
old tickets, so I can purge some invalid user accounts by shredder) and
to my INBOX arrived a bunch of "Undelivered Mail Returned to Sender"
bounces (routed to me through rtadmin alias).
These was transactions notifications for owner of old ticket, but the
owner left our company and his email address is no longer valid. His
account is disabled.

Is this bug? Have I some misconfiguration somewhere or should I besides
disabling user also delete his email address from his account?

Best Regards