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It seems what people *really* want is markers for messages from Amazon, Facebook, PayPal, etc.
But those images may be trademarked, or part of "trade dress", may come with specific usage policies, etc. I'm not a lawyer nor do I employ any, so I'm staying out of it.
Just to let you know that not everybody wants to see those icons in the message list.
Personally, I switched off the GMail-ish letter highlighting in Aquamail, and would've done that in Gmail, if I could. (I have one gmail account that I keep separate for historic reasons, - "attached" to Gmail app.)
So, while I understand the motivation of those people, not everybody wants that.
And just to give one more reason for not doing that: What are you going to use as a "dna string" inside the message? "From:" field? "received by" ? ...? Can you make sure that the message is not spoofed?
What if the message from an employee of the respective company? So, the logistics (and potentially legal) complications are numerous).
