I receive e-mails from various senders, and when I view them in Google Inbox, either on inbox.google.com, or on the Android App, some have an icon in a circle next to them... others just have a letter.
BitBucket, for example, has a nice icon when they send me an e-mail:
The question:
Does Google Inbox fetch this picture with some kind of G+ magic, or are certain e-mail headers involved here?
How do I implement this in my own solutions when sending out e-mails?
The official solution is to reference a Verified Mark Certificate (VMC) in your BIMI record. This handles all of your email addresses at once. Your logo needs to be registered as a trademark, and you will need to purchase a VMC from DigiCert or Entrust. The cost to register a trademark is roughly $660, plus attorney fees, and the cost to purchase a VMC is $1,499 per year.
However, there is a free alternative solution for anyone who can't afford a VMC.
Do not create the Google account using the "To manage my business" option. This account type isn't able to change its profile picture, and the account type can't be changed after its created.
For everyone else:
First, you will need complete the BIMI specification. Then, you will need to complete any additional requirements that each email service provider may have to display the logo in their inbox. This tutorial walks you through the process and explains everything you need to know.