@ilyess
PM supports PGP and automatically enables it, if the recipient uses PGP too and you have his public key, or if the recipient uses PM too.
support@quad9 does not use PGP, so email goes out not encrypted (otherwise they won't be able to read it), and PM uses that fact to automatically scan email for spam.
After the email is sent, they store the email encrypted in your Sent folder and at no point their employee can read the email (so they say, and I have no reason to doubt).
Btw, I tested my guess by sending this email to my own email address elsewhere (also blocked), by sending to PM address (encrypted, delivered), and finally sending to my own email elsewhere but changing the link by removing TLD (the last dot and the part after it) - delivered.
@david @omicron Wait, ProtonMail (PM) didn’t let you send an email because it contained a suspicious link? I thought PM was E2E encrypted and had no visibility into email contents. Was the link part of the email subject?