MS Outlook displays base64 img tags for a while, but then at some point, Outlook fails to render the img tags and shows only a black square. Below is a gist of a sample html file that I'm using as the body of an html email which demonstrates this. When Outlook fails to render img tags, it then cannot properly render any email with base64 img tags. Outlook seems to have an internal rendering class or service that crashes and Outlook must be restarted to recover its rendering ability. It seems that resizing the outlook window can trigger the failure, but that is not consistent.