Check your settings. There is an option 'Automatically enable encryption when possible'.

Hi christ1, thx for your reply. The suggested option is not selected on either account.

In the meantime, I have (theoretically) removed e2ee by setting the option "None - Do not use OpenPGP for this identity" for both identities. The effect was the same, the receiver believes that the message was encrypted or at least that is what the Message Security status button says.

I also removed the option to encrypt the subject, but this also had no effect.

How can I check whether the message was actually encrypted or not? Could this just be that the Message Security status button is being shown based on e2ee history of the identity and not on the actual encryption status of the message?

Thx again for the support.

How can I check whether the message was actually encrypted or not?

You can check the copy of the sent message in your Sent folder.

None of the Sent messages have the Message Security button, so I assume that they were not sent with any encryption or signatures included.

Then I guess that the Message Security button shown by the receiving identity is being shown due to some history rather than what was actually received or how do you see it?

Frankly, I have no idea what's happening at the receiving end. But I doubt that the button is shown due to history.