There's now fewer than 150 days until the 2003 edition of Microsoft's popular server operating system is out of support. Much like the recent end of support for Windows XP, this means that this edition of their operating system will no longer receive any updates (security related or otherwise). Again, much like XP, this will leave machine / servers running this operating systems highly vulnerable to attacks as new flaws in the system are found and remain unpatched.
The final deadline for support is July 14th 2015. Therefore, we recommend that anyone still using Server 2003 should start making plans to upgrade / replace / decommission any servers still running this aging OS (assuming you haven't started doing so already). Servers running this OS will be a prime target for attack - even more so than machines still running XP. Why try and compromise a single machine's email when you can attack the server running several accounts!
The obvious upgrade here is to the much more recent "Windows Server 2012". However, this isn't necessarily the only option. It's worth talking to an expert about what services your server runs and whether this is the best route. In many cases it will be, but it could be that a Linux server could provide a cheaper (and possibly better) alternative. Equally, it could be that a cloud-based service (provided by Microsoft or others) may suit your business better.
See Microsoft's article (and ominous real-time countdown!) about this impending issue here.