Just enter your Mailfence email address instead of the ProtonMail address.
Mailfence can't see into or scan your emails because of their encryption.
Mailfence does the whole business of privacy a little
differently from other services on this list.
First, you will need a Mailfence account, so if you don't have one do that now.
If you decided to use Mailfence, then migrating your contacts over to Gmail is
a very straightforward process.
To setup email forwarding between Gmail and Mailfence, you will follow the exact same steps
as the Gmail Forwarding section above.
Mailfence offers you a complete email suite,
along with features like encrypted email, no tracking or spam, and freedom from surveillance.
On the next screen you will be asked to enter your Google credentials, and
once it's verified your Gmail contacts will be imported into Mailfence.
If you're a Mailfence user then follow the steps below to add
your Gmail contacts to Mailfence and forward any new messages to your Mailfence account.
If you're serious about sending anonymous emails and also want your email communication to remain encrypted,
then your best option is using a service like ProtonMail, Mailfence, or Tutanota.