Setting up an email server is not for the faint hearted. There are however "tun key" options that may or may not suit your purposes.

One such solution can be found here.

If you want a secure email server, then you are looking at a whole different level. However, you could implement email encryption, which would require all users to encrypt their emails as part of the sending process. The problem with that is, they may not bother and simply send the email unencrypted.

Alternatively, you need to channel the whole process through something called a secure shell, otherwise known as SSH. You can find more information here.

The bottom line is, almost all conventional email is not secure. It can be relatively easily intercepted and read.

Don't expect to set-up a secure server in 5 minutes. You'll probably need many hours of reading and understanding to do it properly. And then you've got the on-going issues of security to monitor and address.