PayPal, Amazon, and other large companies set a contact/sender photo in their automatic emails. This looks very professional, however, documentation on how to do this seems to be very limited. To elaborate, by default, most email services have a default contact photo or use say, the first letter of the sender name and have it be the contact photo. I want to use my own.
Is there any single way to cover most services, via an HTML tag or something else? Using a self-hosted Postfix-Dovecot combination.