I don't recall if RT from Best Practical has the particular feature, but otherwise it's a well-rounded and battle proven solution. It's ugly as hell though.
I love RT. Real solid product with great hackers behind it. And you did not mention that it is open source and takes hardly 1-2 hours if you know your way through CPAN and Apache.
The latest 3.8 is not that bad with new theme.
I can even do a free install/setup for any non-profit.
Incredibly powerful, free, customizable. Expect to learn a little Perl if you want to integrate with external systems, as you can write Perl scripts to handle any transaction.
However, it has both the features that the GP wanted out of the box.
In my haste to up vote this, I accidentally down voted.
We've handled a couple hundred thousand support tickets in RT, some of them hundreds of interactions long, and it's still scaling happily.
Using email for support is liberating and using the web interface is phenomenal too. Can't complain about the licensing, but you'll definitely want someone who knows Perl and or the O'Reilly book.
Another vote for RT. It is ugly and looks (or at least looked) really hacky and old, but once you actually sit down and use it, it blows everything else out of the water. RT is the only help desk software that I've used that felt like it was written by people who had spent real time in the help desk trenches, studied the problem up close and actually got the whole help desk procedure.
