Unlimited Domain Emails via Roundcube

I need a straightforward mail-server setup that lets me spin up as many full inboxes as I like under my own domain, strictly for internal company use. I will be creating each mailbox myself, so the focus is on providing the underlying configuration and a clean Roundcube webmail interface where every new account automatically has its own secure password and can both send and receive without issues.

Here’s what I expect:
• Install and configure the mail stack (Postfix/Dovecot or comparable) on my VPS or cPanel server so it supports unlimited mailboxes on the domain.
• Deploy Roundcube, theme it lightly with my logo, and make sure each mailbox can log in independently via HTTPS.
• Set DNS (MX, SPF, DKIM, DMARC) for solid deliverability.
• Document, step-by-step, how I can manually add or delete accounts and reset passwords in the future.