Both mail accounts, ProtonMail and Tutanota are encrypted, and as the knowledge base from ProtonMail states, ‘emails from ProtonMail users to non-ProtonMail users’ are ‘otherwise encrypted with TLS if the non-ProtonMail mail server supports it’.
So, since the email is encrypted at both ends, at ProtonMail and at Tutanota, and is encrypted with TLS during transport, can the email ever be intercepted and read in plain text at any stage in this case?
I've found a site where it says:
"Test mail delivery. Does your mailserver use an encrypted connection (STARTTLS) to send mails to our server? Just send an empty email (email@example.com) and check the result".
And these are the results:
from ***@tutanota.com - TLSv1.3 / TLS_AES_256_GCM_SHA384 - Host: w1.tutanota.de (188.8.131.52)
from ***@protonmail.com - TLSv1.3 / TLS_AES_256_GCM_SHA384 - Host: mail-40130.protonmail.ch (184.108.40.206)
I don't have enough know-how to interpret this, but it appears to me that the mails were sent with encryption.
And for anybody that knows what they are looking at, here are two more pages that seem to have information regarding my question, which I am not capable to interpret myself:
ssl-tools.net/mailservers/protonmail.ch and ssl-tools.net/mailservers/tutanota.com