You were pretty close on the order.
-Contact/Gal
-Gravatar
-Bimi
-Website Icon (pull from root of website, <link ref='icon'>)
-Favicon.ico
-No Image / Monogram
Are you seeing clear examples of BIMI being skipped? That could be a bug
The avatars are cached in 2 seperate 5 day caches.
Contact/Gal (changes instantly, no cache),
Per User: Gravatar
Per server: Bimi, Website Icon, Favicon, and lack of icon (we cache this)
Also, if an email is older than 5 days, we won't update the icon until you click on it in webmail. It won't automatically update in the message list. We did this to keep the load on avatar fetching down.
Matt Petty
Senior Software Developer
SmarterTools Inc.
www.smartertools.com