setting up email

I have began setting up Thunderbird mail. You no longer have to know the security settings of your email provider(s). You just enter your name, username and password, and Thunderbird takes care of the rest. There is one thing that's not explained, once you sign up for a service like this, are your original email accounts closed out after a period of time? Or do you still have manual access to them, if for some reason you need to check in a hurry? Plus, when doing business, there was no Thunderbird email address given to me. So, is this simply a forwarding service? After reading the prior responses, I felt it a good idea to download a email program, but am just beginning with this. Any advice on my questions, no matter how much or little, would be greatly appreciated.

Cat when Thunderbird signs in to retrieve your Email, that is the same things as you signing in each time only Thunderbird is the one doing it.