There is a report that sending emails with either
to ProtoMail has problems.
The other way round works.
(The current version of Gpg4win is 3.1.10, though the version used for the report was not indicated.
- Reproduce problem.
It may (or may not) be feasible to add a workaround for protonmail.
The recipients for protonmail can be detected via their maildomain names.
Then if it is only a small way to use deprecated-no-mime OpenPGP format, it could be selected.
A warning or selection could come up for several recipients.
From the value side:
An argument for adding a workaround is that there are many ProtonMail users,
which value privacy. And it is hard to support the real standard OpenPGP/MIME with a web client.