Apple Mail is trying to use certificate and won't send without I created an account on a website and they installed a certificate on my machine for authentication. I'm not sure how this technically works but the certificate is somehow linked to my GMail address.

Now, when I want to write an e-mail from Apple Mail (from my GMail address), Apple Mail asks me if I want to use this certificate that I mentioned above.

If I say "allow access" it sends my e-mail just fine. However, I don't want to use this certificate to send e-mails but if I deny Apple Mail to use this certificate, nothing happens. It doesn't send the e-mail, it doesn't give me an error message, it doesn't even put it in the "Sent" or "Outbox" folder in the sidebar.

I can see the certificate in my Keychain Access under "My Certificates" if that helps.

Can anybody help me with this?

Thanks a lot!
I just noticed that there is a new toggle button available in Apple Mail when writing a new mail that lets me send the mail without digitally signing it: