It is often useful for a relay to be able to propagate the original sender IP address to a remote relay so that it can apply more intelligent policy rules to the message. The XCLIENT SMTP extension specifies a protocol for doing exactly that. The XCLIENT configuration option allows you to specify whether Momentum should use XCLIENT when the remote host advertises support for it.
Configuration is similar to TLS configuration, with possible values being no to not use XCLIENT (the default), ifavailable to use it if it is advertised and required to prevent delivery if the remote host does not support XCLIENT.
Warning
Setting XCLIENT to required causes Momentum to deliver the mail only if the remote client supports XCLIENT.
The following is an example configuration in the ecelerity.conf file:
binding "customer-1" { XCLIENT = "ifavailable" domain "relay.example.com" { XCLIENT = "required" } }