When the e-mail i sent from the Mira-system, it is sent to a mail server. You can say that it ”leaves the system’s hands”. Thereafter it is sent from the mail server to the receiver. So the problem is actually not within the Mira-system.
It may be that person’s e-mail spam qualifies the e-mail from the system. When you send the e-mail from your own e-mail address, the person’s e-mail address does not spam qualify the e-mail.
If you have Mira’s latest version for e-mail campaigns, you can follow up this e-mail return via the ”E-mail” -> ”Mail Returns”.