Our support staff ran across multiple support tickets where
stopped receiving emails from us and it turned out to be their server's configuration issue with Spamhaus who is preventing access to
their free blocklists using a public DNS.
receives emails for users who cannot receive emails on standard SMTP port 25 and then forwards the emails to their email server on non-standard ports
such as 2525, 26, 587, etc. Other users may also use the
to run their emails through our spam filter system.
The problem happens when they also use Spamhaus on their own mail server and Spamhaus blocked their access.
The error message Spamhaus returned was also misleading so the users cannot identify the issue without digging deeper into it.
Below is the error that our server has received when delivering emails to the email store/forward user's server.
550-Rejected because 18.104.22.168 is in a black list at zen.spamhaus.org
550 Error: open resolver; https://www.spamhaus.org/returnc/pub/22.214.171.124
Note that our mail server IP 126.96.36.199 is not on any blacklist and the real reason is that Spamhaus blocked access to
their free blocklists from the user's server.
Details of the error can be located at
According to Spamhaus, to successfully query Spamhaus via public/open resolvers,
The users need to sign up for the
free Spamhaus Data Query Service
and make the relevant change to the server configuration.