@tengence

Yes, don’t run your own mail-server. That’s rule number one, because it’s difficult to get right and requires a fair amount of maintenance.

If you still want to do that, then I would recommend Dovecot for IMAP, since it’s very fast and use little resources.
https://www.digitalocean.com/community/tutorials/how-to-set-up-a-postfix-e-mail-server-with-dovecot

And if you want to how many email accounts, then maybe store the accounts data in a database:
https://www.digitalocean.com/community/tutorials/how-to-set-up-a-postfix-email-server-with-dovecot-dynamic-maildirs-and-lmtp#dovecot-setup

Both guides are older, so might not be a 1-step in whatever newer OS you might be running.

@tengence

Define “very secure”. Are you talking about stability and backups, or are you talking about someone being able to read your mail?

Google (Gmail) has a very stable system and they’ll take care of backup for you.
If you’re talking about encryption, then ProtonMail might be where you want to look.