SMTP 521 Reply Code

SMTP 521 Reply Code Status of this Memo This memo defines an Experimental Protocol for the Internet community. This memo does not specify an Internet standard of any kind. Discussion and suggestions for improvement are requested. Distribution of this memo is unlimited. Abstract This memo defines a new Simple Mail Transfer Protocol (SMTP) [1] reply code, 521, which one may use to indicate that an Internet host does not accept incoming mail. 1. Motivations Hosts on the Internet have shifted from large, general-purpose hosts to smaller, more specialized hosts. There is an increasing number of hosts which are dedicated to specific tasks, such as serving NTP or DNS. These dedicated hosts frequently do not provide mail service. Usually, these mailless hosts do not run an SMTP server. Unfortunately, users will occasionally misaddress mail to these hosts. Regular SMTP clients attempting to deliver this misaddressed mail must treat the lack of an SMTP server on the host as a temporary error. They must queue the mail for later delivery, should an SMTP server be started at a later time. This causes the mail to remain queued for days, until it is returned with what is usually a confusing error message. 2. Two complementary solutions Two complementary solutions MAY be implemented to deal with this issue. The first one is to use MX relays to bounce misaddressed mails. The second one is to implement a minimal smtp server on the mailless host to bounce all mails. The choice between the two solutions is site dependent.