You recently upgraded to 10.0???? 😱 What hardware are you running? Upgraded from what -- system 9? OS X 10.0 was released in 2001.
Firstly, 10.0 is almost a beta -- it's very buggy, it's very slow, it's very light on features and it's of minimal use. You can't burn a data disk with it. It's not really fit for purpose. Apple had to give away 10.1 because it was so bad.
Secondly, you should be able to run 10.2 or even 10.3 on most hardware that can run OS X. Those are much more polished, useful systems.
Thirdly, you are still going to be limited in what you can do by the age of these OSes and the hardware you have. Forget YouTube or FaceBook. Forget nearly all apps for Mac that you can download at the moment.
As for your Mail problem: have you entered your POP/IMAP and SMTP server details of your Mail server?