Disconnects the mail server and expunging the deleted emails.
Delete method only marks the email as deleted, only the
MailClient.Quit method (POP3 and IMAP4) or
MailClient.Expunge method(IMAP4 only) expunges
the deleted email from server. For POP3, deleted flag will lose if the
connection is closed; FOR IMAP4, the deleted flag is permant even the connection
is closed.