what is the best Mail server to be used side by side with Apache tomcat (which host my main webapp) which I configured to run on port 80? Currently, I have this requirement that I need to setup a mail server under the same server with this address mail.mydomain.com
Any standards-compliant SMTP server should be more than capable of handling a bit of mail. Also note that you don't necessarily need to run the mail server on the same machine (ie "address") as a web application for the same domain. MX records are wonderful things.
Asking "What is the best" for anything is going to get you subjective answers (and likely get the question closed). You have to ask yourself if there a compelling reason to run a mail server as a servlet within Tomcat? Likely there is not, unless for some oddball reason the application cannot communicate with service that outside of Tomcat.
If you need an email server, and assuming you're running Linux, I would say to use Postfix. It's easier than sendmail and has a large userbase. You can use a number of Postfix GUI front ends if you want. Postfix Admin looks pretty good.