This will depend on where your mail was. If it were stored on the server, then you can get it back by reestablishing the account in Mail or some other email client. If it were stored on your hard drive, well... In the future, remember that you can't just delete accounts. You can click on each folder in the Mail sidebar to see which mail belongs to which account. Before you delete an account, you need to create new folders and move the mail associate with that account into them.