Raspberry PI and storage +/- crypto
The built in SD card interface in Razzie is not fit for purpose for any reliable storage (as expected for email). It does the job for hobby stuff, but it is not something I would trust data 24x7x365. It will simply barf after a point. Either the controller or the electrical interface or both.
USB is even worse - any serious activity on it especially combined with network (which also internally hangs off USB) - BOOM. Add to that the fact that Model 3 (which is the only one fast enough to do full disk encryption) will crash thermally if you connect a good USB drive and use encrypted LVM. Write with it for 5-10 minutes at full throttle to a good fast USB disk and BOOM.
If it was some odd job doing storage like a DIY Time Capsule equivalent or DVR, you could have replaced it with a Banana, because that has real disk interface in the form of SATA on the SoC. The Banana is still not good enough for email though - it is a cut-down AHCI, no support for port replication so you cannot hook up two drives in a RAID config even if you wanted to. So you are down to the reliability of a single drive (still better than a razzie though).
That does not mean there are no ARM SoC which can do the job - there are aplenty (used by all those SOHO NAS devices). It is not a job for a hobby SoC though - neither for Razzie, nor for Banana. You need proper disk interface for at least 2 drives and proper NIC interface on-SoC.
So anyone who is trying to sell you a Banana based production secure email device... Just turn around... slowly... walk away... then faster... then run...