That "PreFirstRun" is just a static entry, iow, I have that same entry too and it's only implemented when needed, but mine is in English and yours is in Italian. You can see what it says here:
In English it simply says:
There is no default mail client or the current mail client can not fulfill the messaging request . Run Microsoft Outlook and set it as the default e-mail client
However I do not have any "LastUserInitiatedDefaultChange" entries.
But this is not! the way to solve the problem by you poking in your registry posting keys if you don't know what you are looking for.
Rightfully you need to go back and forth and set each client Outllook, and then WinMail, and click each of them as the default email client - you did that right? ..and SendTo still does not work?
More importantly, did you try installing any Other brand of email clients beforehand???? <..answer question!
As importantly if not more-so, when you first installed Windows 10 and re-instated WinMail, did you First and Only use the instructions on Post # 10 Here ??? <..answer question.
It seems to me you did not reinstate WinMail properly from the beginning - what else did you do?
I do know one thing, if you start over and install Windows 10 and then re-instate WinMail as per Post 10 - then the SendTo will work!
Is that your only problem that the SendTo does not work? Everything else with WinMail works fine?
The SendTo function is very minor anyway, I mean you can manually attach an item within the message instead of using SendTo! I'm not sure you are fully understanding the translation of what I am saying though?
Unless you have Restore Points to go back to, or you have saved a backup Partition Image of your OS and you can restore it back to the beginning and start over, then it just may be if you want to fix it then you may have to start over from scratch with a fresh install, and re-instate WinMail - and SendTo Will work if you do it properly, and then proceeded from there.
My purpose is to serve WinMail installations only - not Outlook. You can get Outlook help in the correct forum. However having said that I have proven quite easily if you install WinMail and Outlook 2016 correctly then the SendTo will work properly for each client, as you saw in my screenshots where I used SendTo on a random file, and it opened the client I had set as default, and attached the file.