Planning to use the replacement with Exchange?
If the answer is yes to that one, forget about replacement, Outlook is the only one. Maybe there is a replacement, but you will not have all the features that Outlooks has. Period.
If the answer is no, Thunderbird (with Lightning plug-in) covers most of the functionality, it is fast and very stable (in my personal experience, YMMV).
Outlook (whichever version) is a piece of crap, nicely wrapped up with a ribbon (pun intended).