OK, I'm not sure this question even belongs here but here goes...
I have an email server running Postfix and Dovecot and a couple of mailboxes there. Using third party email services like Gmail or Outlook I've noticed they fetch an avatars for some emails. Question is how can I attach an avatar to my own mailboxes on my own server such that others will recognize them? Is this a part of mail protocols that I can set up somehow or should I use any third-party services?
Avatars are handled by mail client, not mail server. Gmail fetchs google+ avatars for @gmail.com and outlook fetchs avatar from a Microsoft global exchange address book. AFAIK there is no protocol for email avatars and no easily possible 3rd party integration into gmail or outlook.