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This is probably the way, because a traditional “mail server” is actually 4-5 different servers working together.
- postfix for SMTP
- dovecot for IMAP
- amavis to plug in…
- spamassassin as anti spam
- clam-av as antivirus
And they can all be very easily misconfigured to break everything completely. Great learning experience though.
I would put truenas on the NAS, also put a VM on truenas with 16-24G of RAM.
Create a kubernetes or docker swarm cluster with server 1 and the nas vm and just have everything as containers. This way you just have one resource pool, and the containers will be started wherever there are enough resources available. The containers will mount NFS shares from truenas which truenas will create automatically as ZFS datasets. ZFS supports snapshots.