What does “Message delivered via relay” mean?

In Message Tracking, messages will appear where the subject begins with “Message delivered via relay”. The reason for this is that the sending SMTP server requests a “Delivery Notification” in the SMTP envelope. However, these notifications are not supported by the receiving SMTP server. These notifications are requested via the NOTIFY parameter of the RCPT TO command, e.g.

RCPT TO:<alice@example.com> NOTIFY=Delay,Failure

Depending on the request this must be altered accordingly on the sending side or the receiving side. This behaviour cannot be changed by NoSpamProxy.