I collected some other domains which send me signed mails but had no associated icon in your add-on:

Signed by: Icon:
scv.va https://www.vaticanstate.va/images/favicon.png or https://www.vaticanstate.va/favicon.ico

listen.uni-bonn.de https://listen.uni-bonn.de/ubn-content/favicon.ico

researchgatemail.net https://explore.researchgate.net/download/attachments/557057/atl.site.logo?version=7&modificationDate=1567009946639&api=v2

edv-buchversand.de https://www.edv-buchversand.de/favicon.ico

omnivea.ee https://www.omniva.ee/img/favicon.png

rettet-das-melbbad.de https://www.rettet-das-melbbad.de/wp-content/uploads/2020/08/cropped-i-love-melbbad-32x32.jpg

mail.iherb.com https://s3.images-iherb.com/static/i/favicon-iherb/favicon.ico

jobagent.stepstone.de https://www.stepstone.de/modules/duplo/resources/favicons/stepstone/apple-touch-icon-57x57.png?v=VuWeZv6r

mail.xing.com https://faq.xing.com/themes/custom/xingskin/favicon.ico

e-mai.xing.com https://faq.xing.com/themes/custom/xingskin/favicon.ico

sparkasse-koelnbonn.de https://www.sparkasse-koelnbonn.de/content/dam/myif/sk-koelnbonn/work/bilder/icons/favicon1x.png

loox.io https://global-uploads.webflow.com/5ffa9cb466ed902ac6f8fffc/606492a89e43d4725e0a7a09_Fav.png

mailings.web.de https://img.ui-portal.de/webde/favicon.ico

trustedshops.com https://www.trustedshops.com/static/icons/favicon.ico

groupes.renater.fr https://groupes.renater.fr/static-sympa/icons/favicon_sympa.png

PSU.EDU https://www.psu.edu/favicon.ico