The server can run in several different modes, these are outlined below.
See the Response codes section for more information on responses and which mode responds with which codes.
Accept all email with code 250, 251, 252 or 253.
Bounce all email with a random code, excluding 250, 251, 252, 253.
Randomly accept or bounce all email with a random code.
Server always responds with code 521 - server does not accept mail.