The issue I’m seeing with any free plans of email providers is that they either require an identifying verification, or don’t allow the email to be used for services.
tutamail, for example, states that the free plan is not to be used to signup for services.
protonmail requires verification by email ‘to prove you are human’.
Thank you
Proton and Tuta both offered that as an initial hook, though. Once it became clear that it was no longer sustainable to have minimal restrictions on free accounts, they had to move the needle and incentivize purchases.