There is a report that sending emails with either
- HTML
- attachment(s)
to ProtoMail has problems.
The other way round works.
https://wald.intevation.org/forum/forum.php?thread_id=2089&forum_id=21&group_id=11
(The current version of Gpg4win is 3.1.10, though the version used for the report was not indicated.
Next steps:
- 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.