There are drop-in replacements for Exchange Server, it's just a question of figuring out how to do it...
"DROP IN" means there's a readily available "Downloads" page at some website, where you go to download some 250MB executable file with a free 2-user licence, and once you download that file, you double click on it, and it brings up a choice panel between "Basic" and "Advanced" installation, and 99% of all n00b users can choose the "Basic" installation, and within 5 minutes or so, you're up and playing around with the system and getting a feel for how it works and how your users could benefit from switching to the system.
"Drop-In" does NOT mean that you have to spend a week at Amazon, reading through thousands of product reviews, deciding which stack of softcover programming books with pictures of bizarre esoteric mammals on their covers you will need to purchase, and then spending about three or four years of your life actually reading the stack of softcover programming books with pictures of bizarre esoteric mammals on their covers, until you have a PhD-level understanding of C-Compilers [and a worthwhile opinion as to whether you should compile with the GNU compiler or Clang/LLVM or the Intel compiler or the M$FT compiler or the WTFE compiler], and which libraries you will need to compile against, and the theory of BASH shells, and the theory of prime numbers & elliptic curves and LDAP authentication, and the theory of sector-level hard-drive replication living beneath WebDAV and blah blah blah blah blah blah blah.
And then five or ten years later, when you've finally earned your "PhD" from the "School of Hard Knocks", you've still got to spend six or eight weeks at various internet forums, from Palo Alto to Bangalore, trying to figure out WTF line of code you need to add to some configuration file to get it all to work together reasonably "seamlessly", when finally some poor fat bearded dude, working for free from his mother's basement in Bulgaria, finally points out to you that your corporation can't do this because your 1,000 client computers have a known "issue" involving their 3Com ethernet chipsets in combination with their ATi graphics chipsets, but that no one has been able to chase down where that particular bug might live in the total code package, and so you're SOL.
At which point you learn that no one ever got fired for purchasing M$FT.