How to "attach" an image to letters?

Question

Mail server

Greetings
, I don’t even know how this thing is called correctly, but I want to understand the algorithm of its work and apply it to letters sent from the server. The question is how to attach the desired image to the letter so that it can be displayed in various mail programs (usually displayed to the left of the subject line).
BlueMail:

Mail.ru (web and android):

It is clear that in some web versions of programs, images are taken from the user profile and substituted in the right place, but if there is no registered mailbox in some large service, then where does the image come from?
Many send letters from their servers / hostings, some have their own images next to the letters, while others do not have such images, and mail services / programs themselves substitute something instead of the image, for example, the first letters of the sender's name.
I looked at a dozen different letters from different companies with images next to the letter and concluded that I don’t understand how these pictures are substituted at all. Some have just a picture in the body of the letter, others have a picture with a link to the site, others do not have any pictures in the body of the letter, but the image next to their letter is displayed. I looked through the source codes of letters and did not find any regularity. Tried this way and that, but there was no image next to the letter, and no.
Please clarify the situation.

Answer 1

You can add an icon for the entire domain via
https://postmaster.mail.ru/
or register icons for individual addresses via gravatar:
https://ru.gravatar.com/
many MBPs understand gravatar.

Answer 2

This option will most likely help you .

Answer 3

Mail programs take favicons or other icons, but only for popular domains. If your domain is not even approximately mail.ru, then nothing.