Hmm so after some testing...
Set "no" on "po_active" in "mail_users" has the result, that the user can't login anymore to the account = that's ok.
BUT it also has the result, that someone else can't send any mails to the account:
So this is not, like it should be
The Sender gehts an E-Mail from the server:
Quote from Undelivered Mail Returned to Sender
This is the mail system at host SERVER.I'm sorry to have to inform you that your message could notbe delivered to one or more recipients. It's attached below.For further assistance, please send mail to postmaster.If you do so, please include this problem report. You candelete your own text from the attached returned message. The mail firstname.lastname@example.org>[/email]: user unknown. Command output: Invalid user specified.