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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?

Additional Information

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 (check@ssl-tools.net) and check the result".

And these are the results:

from ***@tutanota.com - TLSv1.3 / TLS_AES_256_GCM_SHA384 - Host: w1.tutanota.de (81.3.6.162) 

and

from ***@protonmail.com  - TLSv1.3 / TLS_AES_256_GCM_SHA384 - Host: mail-40130.protonmail.ch (185.70.40.130)

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

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    As far as I know, an email that is sent from Protonmail to Prontonmail is encrypted at any stage. An email that is sent from Protonmail to non-Protonmail is only encrypted at transport if mailserver supports and it's plaintext on this server. – user215422 Apr 8 at 20:15
  • Protonmail supports PGP with non-Protonmail users, but it has to be explicitly setup. See: protonmail.com/support/knowledge-base/how-to-use-pgp . Without tthis external PGP setup, I would expect clear text conversion at some point on both servers. – user10216038 Apr 8 at 21:39
  • I agree with the other commenters. The post you are referring to is: protonmail.com/support/knowledge-base/what-is-encrypted. It specifically states that most mail providers (e.g. Gmail, Yahoo, Hotmail) support TLS, but this is not end-to-end encryption - i.e. "this means Gmail, Yahoo, Hotmail, etc, will be able to read these messages and hand them over." – mti2935 Apr 8 at 22:39
  • I know about Gmail, Yahoo, Hotmail, ect. I am not talking about them. I am purely interested in what happens between ProtonMail and Tutanota. They are both encrypted email providers, and neither of them is supposed to be able to read your mail.So the question is not what happens on their servers, but what happens during tranport. Are the mails encrypted with TLS on-route if the password protection is not specifically selected? – questionmark Apr 9 at 8:13
  • TLS in email providers is basically plaintext to some attackers. If you want security you need to encrypt the message contents with your recipients public key. Don't trust what a vendor tells you about their level of security, implement something that meets your standards which you have control over. – john doe Apr 9 at 19:54

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