Security is only as good as the weakest link. If you send an email to a non-ProtonMail address, then it is sent as normal emails are: no security but maybe some

Security is only as good as the weakest link. If you send an email to a non-ProtonMail address, then it is sent as normal emails are: no security but maybe some encryption in transit if you're lucky. You can either get your recipient to get their own ProtonMail account (for free!), or use a protocol that was designed respecting security and privacy in the first place i.e. don't use any email service. You can in theory get transparent pgp e2ee using two MUAs on separate PM accounts communicating with PM via PM Bridge. But it's simpler/easier to just use their web front end. No configuration/installation of MUA or Bridge required. At least this way you don't end up with a plaintext copy sitting in your MUA's outbox. In essence the encryption is applied one hop too late (by the Bridge) if you're concerned about the security of your client endpoint or filesystem. This is a good resource: https://protonmail.com/blog/protonmail-threat-model/ https://protonmail.com/blog/protonmail-threat-model/