I guess I'll chime in here too, because no one seems to have any technically correct info in this thread. The post is correct regarding SMTP. But not PGP encrypted emails.  

 PGP encrypted with private keys stored in ProtonMail?
 

 This is another point not mentioned before.
External pgp is stronger than protons in house version via web browser 

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 If it's gmail to protonmail, it comes in unencrypted then they encrypt it