I have a friend in a neighboring state that I visit regularly - we’re setting up disparate SANs, one at his location, the other at mine. We each get half the storage space; we back up to the half onsite and overnight the onsite SAN data gets backed up to the offsite. This has nothing to do with mail, but if you can host a mail server on something as inexpensive as a pi then you could have one at multiple locations for redundancy purposes.
EDIT: I just realized I typed SAN instead of NAS. It’s NAS, I just don’t deal with NAS’s all day at work so I always write that by default.