If you wait long enough, you’ll find an HP 1/8 G2 LTO-5 SAS autoloader tape driver somewhere between $250-$350 on eBay. That gives you 12TB uncompressed or 24TB of hypothetically compressed storage. Then buy the absolutely cheapest SAS card like a Dell H200E (which goes for about $25) and you’re all set. Don’t forget the appropriate SAS cable.

And, this combo works perfectly with Veeam Community Edition.

Why bother with complexity? Just use a mail relay like SMTP2GO and all your potential problems will be solved regardless what self hosted email server solution you choose.