How Do I Add a Graphic To Email Subject Lines?

I really appreciate the great work you do and how clearly and simply you explain things. I’m interested to know how you create the “apple” icon in your email subject lines. I love that effect. I can instantly recognize your emails from the crowd.
I wondered if I might be able to create this same effect for our company emails using the icon from our logo. Note that we do use Outlook – so it may not be feasible. But if it’s possible to do in Mac Mail, I would use it for personal emails with my own “branding” symbol.
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Grant