Free selectable recipients

I'd like to see to have the choice of selecting recipients not only from a pre-defined category list but also have the possiblity to select single users.
Example: User A, B and C are in category 1. User X,Y and Z are in category 2. Now, I'd like to send a mail to user A, C and Z but NOT to user B, X and Y. So I could do a third category with the desired recipients in it but the next time I want to send a mass mail to other recipients the whole story begins from scratch. Also non-admin users do not have the possibility to create categories at all.

It would be cool if one could see a list of all recipients of a chosen category on the mass contact creation page. All recipients have a checkbox at the left side which for default is checked. Now I'd like to have the possibility to check/uncheck either single users or check/uncheck all of this category (unchecking all should be provided for starting from scratch with checking users).