NTFS.
NTFS is best suited for managing thousands of files, which is what you do with SmarterMail.
ReFS, on the other hand, is suited for managing VM workloads (i.e., VHD/VHDX) and databases like msSQL, or backup storage.
I'll also add one thing: although ReFS "should" be more secure and stable in terms of data preservation, we've already had strange errors that made it impossible to read entire VHDX files (while others remained readable ON THE SAME PARTITION...).
With NTFS, this has never happened to us...
In practice, NTFS is much more mature and stable (ReFS is NOT ZFS, even though Microsoft hoped so...)