encryption – Are emails sent from ProtonMail to Tutanota (or vice versa) ever not encrypted?

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?