That's fine...

...if you are only ever going to receive emails from the UK and everybody that you're ever going to exchange messages with 1) has no objection to getting clipped by the Post Office (you think they'd do it for free?!) and 2) that you don't mind handing your home address and phone number to everybody you email.

In my work, I find myself exchanging emails with complete strangers on a frequent basis and I'm damned if I want to be adding my personal details to every exchange. Most of the time it would be fine; but just occasionally....... Plus, the Direct Marketing Association et al would definitely be early adopters of the certificate reading gear, because then they could hit your landline and snail mail for their campaigns too.

All such a certificate would prove is that "person X had access to Photoshop and £30 (or whatever the registration fee is) at one point in their lives".