First I'm not sure What the win10 mail app is web based, or PC client based as I have never used it on my win10 system
The Mail Shield signature only works for email clients (like Thunderbird in my case), as this signature is generated on the system prior to being sent over the normal email ports, not Web Mail where email is compiled and stored on-line.
I think it's a client, it's basically a simpler Outlook.
It's working in Gmail website though...
Edit: Gmail is also working on Windows 10 Mail app, so.. the issue is email provider related, not the client. What could it be? Strange.
A very simplistic explanation is a Client based email program, stores copies of sent and received emails on your system
If it is the same as Outlook, that uses a Avast add-on actually within the program to handle email scanning (and presumably adding signatures) not the Mail Shield.
My problem is:
It is a very, very long time ago that every used MS Outlook before I went to Outlook Express and finally to Thunderbird.
I have never used Microsoft's Windows 10 Mail, I thought I would give it a quick try to see what it was. But MS forces you to create an MS Account, so that was the end of my trial.
There are many that no longer use the signature addition, because it could be faked in a spam/scam email and some feel it advertising rather than confidence that the email actually is clean.
Normally Gmail is web based email, viewed through your browser and email stored on the Gmail servers (not on your local computer), but could be handled through an App.
If the email is compiled (via an App) but actually stored on an on-line server (MS or Google), I don't think Avast would be able to add a signature as the source code is isolated from the local system.