Cybercriminals have spammed out malware posing as an email from the Royal Mail. The emails claim to come from official-sounding addresses such as customer@royalmail.com. Opening the attached file could lead to your Windows computer being infected by a Trojan horse. The reason given for non-delivery of the parcel can vary. For instance, the email might claim that your address does not exist, or that the parcel is too large. For example, the reason given is that a courier did not deliver the package to your address.”]
Source: https://grahamcluley.com/that-package-at-the-royal-mail-office-its-malware/