Would you be able to share some stats, like how much disk space these accounts take up and such?
For larger scale e-mail sending I built my own MTA (https://github.com/zone-eu/zone-mta), our main instance sends about 750k emails a day (mostly normal ISP traffic, maybe 25% marketing emails) and its most valuable feature is juggling with IP addresses and blacklist detection, so if some mailbox gets hacked, starts sending out spam and the IP ends up in Spamhaus et al then this IP is removed from the list for other users automatically. Has saved us a ton of time.
Sure, the mail storage currently takes up 1.01TB, using dovecot's mdbox. This mail store started in January 2016, after the service got raided twice by german authorities[0] (at request of u.s. gov), otherwise it would be larger.
I'm not going to run like per-user stats but I know historically there's been about 11kb per E-mail on average, so that's about 90 million mails stored. The MTA itself sends and receives a bit over 20 million E-mails annually, so the extra from that is probably from the mailing list I run on the same server (where each message only counts as 1 for statistical purposes)
How I prevent outgoing abuse is a black box, but I do it well enough that mail from my server almost never ends up in spam. But given that it's a free service I definitely don't have the budget to warm up and dynamically scatter mail across IP space to maximize deliverability. Cool technique, though!
Oh, btw, the easiest way to test ZoneMTA would probably be to install the WildDuck mail server to some empty VPS. The install script, amongst other things, installs and configures ZoneMTA https://github.com/nodemailer/wildduck/tree/master/setup
For larger scale e-mail sending I built my own MTA (https://github.com/zone-eu/zone-mta), our main instance sends about 750k emails a day (mostly normal ISP traffic, maybe 25% marketing emails) and its most valuable feature is juggling with IP addresses and blacklist detection, so if some mailbox gets hacked, starts sending out spam and the IP ends up in Spamhaus et al then this IP is removed from the list for other users automatically. Has saved us a ton of time.