Been a while since this was fixed now, but I once discovered a method to track views of even plain text emails when the user was using Thunderbird - https://www

Been a while since this was fixed now, but I once discovered a method to track views of even plain text emails when the user was using Thunderbird - https://www.grepular.com/DNS_Prefetch_Exposure_on_Thunderbird_and_Webmail https://www.grepular.com/DNS_Prefetch_Exposure_on_Thunderbir... - thanks to the DNS lookups caused by URL pre-fetching. Same issue worked with various webmail implementations at GMail, Hotmail, Roundcube, IMP, and probably more. You can test your client for this particular flaw and many more at a website I built - https://www.emailprivacytester.com https://www.emailprivacytester.com