The Via: header is not defined by RFC2822 but occasionally appears in mail messages that sendmail needs to process. It is used by a few other networks to mark a mail message's transit through a forwarding host. It is an early, and now obsolete, version of the Received: header. The sendmail program counts the Via: header when determining the hop count but has no other use for it.

The Via: header should never be declared in the configuration file.