Support Start > Email > Spam and Virus Filters > Spam Filters > Domain Not Found Email Rejections

If someone sends you mail from an address like “address@x.y.z.example.net”, and “x.y.z.example.net” is not an actual “hostname” in the Internet’s DNS system, our spam filters will delay or reject that mail.

The fact that it’s not a real hostname means the message violates Internet mail standards and appears to be forged. This is something that only spammers usually do, so our servers (like most other mail servers) will consider it suspicious.

The proper solution in such a case is for the sender to either add “x.y.z.example.net” to the DNS system, or to change the mail sending address to match a hostname that already exists. If that can’t be fixed, though, and you really want to accept mail from such a sender, you can add their address to your whitelist.