anecdotal, of a number of people I know who run the SMTP (and IMAP, etc) infrastructure for some mid to large sized ISPs, none of them are protonmail customers.

anecdotal, of a number of people I know who run the SMTP (and IMAP, etc) infrastructure for some mid to large sized ISPs, none of them are protonmail customers. email was never meant to be secure in the way that they market it as.