Mail Server

Hello,
I'm moving some sites that my friends and I handle from a cheap
webserver to a cloud server with OpenBSD.
I have the webserver part figured out (httpd+mariadb+php7).
But I need to choose how to handle the mails.
I'm mostly worried that the mail accounts must be managed by some of
our users through the web.
So I need to use something like postfixadmin or ViMbAdmin, that is, a
web application to handle the accounts.
Also I dont have much knowledge about what the market has to offer...
Should I use OpenSMTPD or I need some big MTA like postfix ?
I also need IMAP so can I use OpenSMTPD+dovecot+ViMbAdmin?

This is a kind of open question, maybe someone savy in mail servers
can guide me in what to choose.
The amount of mails in and out is very small, so, I'm not looking for
performance, just for security, reliability and, truth be told...
practicality, to many components to handle a mail... It should be
simpler... ;(

Cheers.
Elias.