I noticed that the review just passed over opera's mail client.
As a user of it for a few years now, i appreciate this program a lot. So let me explain a bit.
opera handles mails differently: it considers the mail base as a data-base, and that's what it is after all.
Each mail is a record in mail file. The mail is accessed via indexes like a record in a file. one can create as many indexes as necessary and of any kind. The usual filters are replaced by indexes. The first advantage of this system is speed, as mails are no more moved around. the second avantage is space, because to see a mail in 2 places one has 2 indexes, not 2 mails. when one creates a new contact, it is a new index and you get immediately all the mails from an to this contact. default indexes of opera are the contacts, recieved, sent, spam, trash, active threads, unread, joined files. then one can add new indexes simple or conbinated. once used to it, it is incredibly powerfull.
the negative point: one can't create html mails. contact list must be exported to be used by other programs.
Very often people are too lazy to give such a different program a chance to show how natural it is to work with, just because they are stuck in their habits. this one needs to do that effort, but reward is great!