Email server - where to begin?

HI, I work for a small business, and up until this point we have been using services of a local contractor to host our email. However, they are not the greatest, and just to be able to constantly have an eye on our email (to be able to upgrade it if need be, maintain it constantly, and ensure confidentiality is up to the highest standards), we would like to set-up an email server in-house, as we are already planning on getting a server for different purposes. I have previously worked with Ubuntu, and reading through some of the tutorials on here I am more confident that we will be able to take on hosting our email server. However, I am not sure where to begin; which server will be enough for 30-50 users; which redist I should use; or even if mail-in-a-box is the right choice for our needs.

I would really appreciate if someone could clear it up for me a little bit.

Cheers, Magic