Help Teach Me How To Setup Secure Email Server For Ubuntu 14.04.1 LTS With Support For Multiple Domains + LAMP Advice

I'm a software developer looking to learn how to setup my own email server that can support multiple domains and then setup a webmail like roundcube to access it. I know how to install the following on this version of linux :: Ubuntu 14.04.1 LTS I know how to program very well in PHP, use MySQL etc... I've installed... - Apache2 - PHP5.5+ (With module for Apache2) - MySQL What I'm really struggling with is setting up an email server. I was able to successfully install Postfix & Dovecot (which I prefer unless you can explain to me that advantages of the competitors like Courier). After I installed this I installed the webmail roundcube and got it to work, thing is I only know how to make it work with 1 domain name, not multiple domain names. I can create users on the system and the webmail assumes its their username[at]mydomain.com... but what I really want is username[at][login to view URL], username[at][login to view URL] etc..., because I plan to host many websites on this server with different domains. I'm also need help with how to protect my email server from being abused, basics of firewall installations and security etc... Looking for a server admin expert basically to teach me the ways :). I can give you lots of work related to setting up servers in the future if I like the way you setup for clients I get from time to time and your rates are reasonable. For a professional that knows this I don't think this should take more than 1 to 2 hours, so please quote accordingly, thanks!