https://forums.adobe.com/people/Nancy+OShea wrote
Who is going to be receiving these HTML e-mails? Are all recipients on your corporate LAN? And which e-mail client are you targeting?
Excellent question - there is a difference between internet and intranet. Both require a server but only the former requires a service to get a URL. The latter doesn't need a URL since it would would know what the internal IP address is. (Not blocked by a firewall)