The sender is blank in the Track and Trace logs/results or doesn't match when the email is received.
Email security cloud
The Envelope Sender is used for SMTP routing while the Body Sender is what appears in the "From" field of a mail client.
Track and Trace logs show the Envelope Sender, not the Body Sender.
Some companies or individuals configure their email systems to use specific Body From headers that are not equal to Envelope From. Distribution Services often use long, unique Envelope Senders to manage bounces, which differ from the visible "From" address.
Most often, mail clients will provide the Body Sender information as the sending address. For distribution lists in particular, the Envelope Sender can be lengthy and can confuse or alarm the recipient. In addition, it will be most likely dismissed as spam.