If you Google "squirrel mail and postfix" or "squirrel mail and dovecot", you should find some guides that can walk you through setting this up yourself.
Squirrel mail is like an online email client where you read your email, like Gmail.
Postfix/dovecot are the bits that actually send and receive mail in the background.
I've done this before though and it was SUCH a pain. It takes ages to set up. You have to be very careful to secure it properly. You need to configure several anti-spam measures (like postgrey) and setup your domain with DKIM and SPF. And most people you send email to will block you outright at first. You literally have to write an email to someone at Microsoft to manually whitelist your server's IP address before anyone with outlook will receive your mail. Until then, email won't at least even get sent to their spam folder - it'll never even get that far!
These days I flat out refuse to go through all that!
I set up my clients with Google's "G Suite" a.k.a "Google Apps for Work" a.k.a. "Gmail for your domain". Its a low monthly cost per user. Everyone is used to Gmail and sharing documents and calendars within an organisation is super nice. (If you're interested, send me a forum PM and I might be able to get you a discount on it as well if you're in the UK)
That all said, Squirrel Mail is quite nice for internal organisation email.