[jowsie]

Searching for '"AD friendly" email' brings up another temporary email service that this appears to be a clone of (https://huo.fo/), and this hackernews post with you asking what it means.

Active Directory comes to mind, but I'm not really sure what that would entail.

[supriyo-biswas]

It's more likely that whatever open source solution they are using to run this, allows you to configure ad insertion easily.