It is possible for an application to write to disk by bypassing certain file system interfaces - especially if the company making the software makes the operating system and the file system, such as Microsoft. Whether or not MS Exchange does something like this I do not know.
Another matter entirely is whether or not a product is supported using NAS (technical support, I mean). It could be that everything will work fine, but if you ever have a problem the company will tell you that you have an unsupported configuration. This is because NAS might not have a way to guarantee ordered delivery or acknowledge receipt of all I/Os. If you have a transaction-based system, these are important to maintain the integrity of the data. NFS, being stateless, has no way of providing this.
This was first published in June 2001