In some horrible world where Murphy's Law applies, what could go wrong (i.e., privacy subverted) even if you use ProtonMail?
Pretty much the standard culprits:
- Unknown browser vulnerabilities could render the encryption useless (ProtonMail is entirely web-based)
- Your computer could be compromised by malware (keyloggers, e.g.)
- Your recipient's computer could be compromised
- Unknown weaknesses in the encryption render it useless