Seems a bit nearsighted to accuse every service of malice and then completely ignore that tutanota fixes lackluster pgp encryption by also encrypting the subject line.
This works virtually identically between both providers, except that Tutanota encrypts both the message body and subject line, whereas ProtonMail only encrypts the message body. This doesn't pose a huge risk if you use the former service. Just make sure that your subject lines don't contain any sensitive information. source
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