Some email providers allow you to actually close an account but many (probably most) don't. All you can do is to stop your PC from downloading the mail. i.e. if your account is
[email protected] people will still be able to send mail that address but it will just sit on the server at someprovider.com. (Eventually your mailbox, on that server will fill up and someprovider.com will start sending 'bounce' messages to people who use that address).
The number of email accounts I've used over the years runs into many hundreds. (I frequently open a new account just to receive messages relating to a single post here on AB). However, I'm only currently receiving mail from just 4 accounts. (Anything sent to the other addresses never reaches me). As long as you're happy to simply stop receiving mail (rather than actually closing the account), here's how to do it with Outlook Express:
Go to Tools > Accounts > Mail
Click on the name of the account you want to delete.
Click 'Remove'
I've never used Thunderbird but the procedure is probably similar to that for Outlook Express.
Chris
(PS: If you require a new email account, accessible through Outlook Express, Thunderbird and the web, see paragraph 3, et seq, here:
http://www.theanswerbank.co.uk/Technology/Inte rnet/Question407429.html
You can have as many accounts as you like)