Removing a mail account removes the account information from Mail and deletes the account’s mailboxes and messages that are saved on your Mac.
Warning: Deleting a POP account permanently deletes its mailboxes and messages from your Mac and cannot be undone. If you back up files using Time Machine, you can use it to recover previous versions of mailboxes.
Removing an Exchange or IMAP account doesn’t permanently delete the account’s mailboxes and messages because they’re stored on the mail server. To retrieve the account’s mailboxes and messages from the server, add the account again in Mail.