Accidentally deleted email account

If you had disabled to option to delete mails from your server, you should at least receive the mails still stored on the server by setting up your old account again in Mail.app.

Deleting a mail account causes all your mail stored in that account’s mailboxes to be removed from the computer as well, and you were warned that this would happen by the following alert:

Remove Account
Are you sure you want to remove the <AccountType> account "<AccountName>"?
This will permanently delete the account setup information, mailboxes, and messages from your computer. Messages stored on the mail server will not be affected.

For IMAP-type accounts, this is not a problem because mail is stored on the server. If you deleted a POP account, however, your mail has certainly been wiped out from the computer, not just moved somewhere.

To prevent that from happening, you should have moved your mail to custom “On My Mac” mailboxes instead of leaving it in the account’s mailboxes. Actually, using the account’s Inbox and Sent mailboxes for archiving purposes is a bad idea regardless.

As m.bode said, if the POP account was configured to leave the messages on the server for some time, setting up the account again would cause any messages still on the server to be re-downloaded, but this would only work for received mail that has not yet been removed from the server.

If the messages are not on the server and you don’t have a backup (which you should have made before doing something like that), you may try to salvage as many deleted .emlx files as possible with a data recovery tool such as Data Rescue II or FileSalvage (the files to be recovered would be different in the case of Mail 1.x). Stop using the computer right now if you want to try that, as anything you do with it may cause the deleted files to be overwritten.