This specification adds the 521 reply code to the repertoire specified in SMTP, reserving it for use at connection-opening time to indicate that the host does not accept mail under any circumstances. It SHOULD be used for dummy SMTP servers whose sole purpose is to notify systems that attempt to open mail connections that the host never accepts mail. It MAY be used in other situations where the intent is to indicate that the host never accepts mail. It SHOULD NOT be used for situations in which the server rejects mail from particular hosts or addresses or in which mail for a particular destination host is not accepted. As discussed in SMTP, reply code 554 is more appropriate for most of those conditions; an additional case, in which the determination that mail is not accepted is determined outside the mail system, is covered in the next section (Section 4).
“Server does not accept mail” (or a variant such as “Host”, “Domain”, or a related term) is an acceptable message to accompany a 521 code used for this purpose.
Once the 521 reply code is returned instead of the usual 220, the SMTP session proceeds normally. If the SMTP client attempts to send additional commands other than QUIT, the server MAY either continue sending 521 reply codes or simply close the connection. If the purpose of running a dummy SMTP server that returns a 521 code is to conserve resources, the latter will usually be preferable.