Email arrives on smartphone but not on PC

Hello,

It happens, with the same sender of two emails, that one is visible on my smartphone (or both) but does not appear on my PC. A few days ago, the tracking information of a registered shipment from the post office site arrived the same on the phone and not on the PC. Moreover, after forwarding the mail to my own address, the drawback was that the attachment was no longer there. Fortunately, I manage to find the data on the sender's site. But what is the cause of this different availability and how can it be fixed?

Computer: HP with Windows 10 or 11 via Free. Smartphone Samsung Galaxy A.

Thank you for your help.