Sender DomainDNS


One class of connection rejections that happens later than any others here are ones based on the domain part of the SMTP MAIL FROM command (the sender domain). Just as we do with connecting IP addresses, we require that the domain of the message sender exist in DNS, and we also require that it at least appear to accept inbound mail. The driving force behind these policies is that we don't believe we should accept mail to which our customers cannot reasonably reply, if need be. Below are the error messages that will cover these rejections.

  • 554 5.1.8 - Domain of sender address SENDER@DOMAIN.TLD does not exist - This error message means that we received a response of NXDOMAIN when trying to resolve the domain of the sender address.
  • 421 4.1.8 - Domain of sender address SENDER@DOMAIN.TLD does not resolve - This error message means that we received a response of SERVFAIL when trying to resolve the domain of the sender address. This generally means a transient failure in DNS either with the servers that are supposed to answer authoritatively for the domain in question or with our ability to reach them.
  • 421 4.1.8 - Domain of sender address SENDER@DOMAIN.TLD does not accept inbound mail - If the domain of the sender address is publicly advertising the fact that it does not accept inbound mail, then we will not accept mail from that domain.