That configuration should be more than ample for your Users and Email Volume. We have about 300 domains and 3000 users (most of which are IMAP) with a daily email volume of 60,000 Incoming/30,000 Outgoing messages running on the following:
Quad Core X3450 - 2.66Ghz
Windows Server 2008R2 (will be upgraded to 2012R2 next month)
16GB ECC RAM
4xSeagate Constellation ES.3 SAS Drives (Raid-10)
Dell SAS6/iR Raid Controller
Very similar to your configuration (with half the RAM and physical HDD) and we still have plenty of headroom to grow (we rarely peak over 10% CPU).
Although we aren't running SSD drives on this server, we run them on our SANs and in our ESXi boxes with much success. They are as reliable as traditional Enterprise HDDs and we use them without hesitation for Web Servers. Personally I'm still too overly cautious to run them in heavy I/O environments such as for SQL Servers and Mail Servers, but many others here have used them successfully in their Smartermail installations without trouble or complaint .