We use NFS here (yea, I know) but it's on a 7-node storage cluster over GbitE. Honestly around here, email traffic is nothing compared to the 2T files floating around.
If you're doing that much traffic with email, you might put a lot of load on your network if you use NFS. If you can, keep your data local. Oh, and I wouldn't really recommend a single 750G drive, you want something with RAID for redundancy.
And yes, avoid windows. Firstly, CIFS is nothing like NFS, especially when it comes to file permissions. And... if you really care about your email you don't want it at the mercy of a Windows box... that's why you're using ZCS
