Just trying to bump start some interest in this again.
SnappyMail has integrations for favicons.
Note: If you have not published an email address (e.g. because you don't want it spammed) avoid using that email address as the seed for the identicon, as it can be extracted from the image.
https://jdenticon.com/ is the address of the project it seems.
One of these two things does not fit to the other however:
avoid using that email address as the seed for the identicon, as it can be extracted from the image.
An Identicon is a visual representation of a hash value, usually of an IP address, that serves to identify a user of a computer system as a form of avatar while protecting the users' privacy.
But overall https://jdenticon.com/ looks like a project that can be integrated into an project, but does not really seem to be something to run standalone as a cloudron app.