With most e-mail programs, including Outlook, Outlook Express, Eudora, and Entourage, deleting an e-mail "account" in the program does not delete the messages that have already been retrieved. You can safely delete or replace an e-mail account in these programs without affecting or deleting previously downloaded messages.
However, the Mac OS X Mail and Thunderbird programs work a little differently. Read the information below before deleting accounts in those programs.
With the Mac OS X Mail program, when a mailbox (account) is deleted, it removes all messages associated with that mailbox.
Before deleting a mailbox in Mac OS X Mail, first move the downloaded messages into another folder or to the Inbox. That way the messages will still be in your Mail program after you delete the mailbox.
If you use Thunderbird and originally set up the account to use the "global mailbox", then it's okay to delete the account. However, if you set the account to save the messages into its own mailbox, then deleting the account will also delete the messages.