TL;DR: Paypal is embedding an image into its emails using the "//" shortcut, instead of specifying http: or https:. This is wrong since it assumes the mail use

TL;DR: Paypal is embedding an image into its emails using the "//" shortcut, instead of specifying http: or https:. This is wrong since it assumes the mail user agent loaded the content from a web server. There's zero benefit to dropping http: or https: in a URL in an email.