How do we get sender icon for emails in any email client (so ios mail for example )

Question

In emails these one sees companies using icons/ avatar showing up in email sender (without opening the email) .. similar to what favicon does in browser tabs, how do we set that up ? (see Medium, Wework. Brightalk example for below screenshot)

Most answers on the net seem to specific to Gmail and we are looking for a generic solution.

Answer 1

There's no standard for this. Every app has their own ideas and is often based on a proprietary or OS-specific address book.