I use Thexyz Premium and have for a while. It is a great product and I love the ability to use pseudonyms but the user interface is lacking a lot in usability. You can set a rule that puts incoming mail from a specific sender in a specific folder but you can’t set a rule that will put mail to a pseudonym in a particular folder. Nor can you send mail from a specific folder — every sent mail gets put in the sent folder even if you are responding to a message that is in a folder. So in order to keep track of incoming and outgoing messages you have to keep moving them around.
It seems like Thexyz got halfway to creating an excellent product then stopped. I have emailed Thexyz a couple of times and received responses indicating they thought my ideas for improvements were good but nothing is done.