MIAB uses “+” as a delimiter for “recipient_delimiter” in postfix.
Otherwise it looks like it should handle everything you need it to.
Yes, MIAB handles multiple domains at a time. as many as you need / want.
MIAB does handle all certs for you so you don’t need to touch them (unless the website you host is not hosted in MIAB). only 1 ssl cert is needed per domain if miab is doing all hosting for it (DNS, Mail, and Web Hosting)
IMAP support is in MIAB as well as ActiveSync.
Another solution is mailcow, iRedMail, Zentyal, etc. (Since I used to use mail-in-a-box, I have since moved to Mailcow, both MIAB and mailcow work VERY well.)