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Unfortunately for providers like ProtonMail it doesn't detect or show anything. Most automated mails also don't have anything in there, which is okay but it leaves a blank space in the header that looks ugly when this happen. There should be a placeholder icon when it can't detect anything.

Tato recenze je pro předchozí verzi doplňku (2.9.13). 

Yes, I agree. I've been thinking about what kind of distinctive icon to use for this. Maybe you could propose something through a GitHub issue at https://github.com/marcelstoer/display-mail-user-agent-t.