Depending on the software you use to interface between the PC and the ISP, the internet isn't 'always on'. I have WinPoet, and in fact that disconnects you for idle time. Read thru the ISP/modem software options, or call their help desk.
There are 2 ways to 'disconnect'. 1) mIRC disconnects or gets disconnected from the server... 2) Your isp/Internet connection disconnects. If #2 is happening, then this isn't a mIRC related problem. Simply looking away from mIRC to get your email certainly shouldn't be a consideration as to why a server would drop you. Try other servers.
I have DSL on a network, I will leave my pc and even my house for days at a time and Im still connected. Chances are, you have to tweak your own pc/connection for the answer to your problems. While we can point you in the right direction, there isn't a simply answer that fits everyone's pc/ISP setup, it's going to take research and reading on your part.
