A simple real-life question, inspired by a comment here:
Active Directory has supported inter-site replication using SMTP instead of direct RPC since its first introduction in Windows 2000.
But did anyone ever actually use it?
If yes, why was it choosen?
Was it easy or troublesome to set up and mantain?
Was it reliable?
The reason you don't see it, and probably never will, is because it was designed to support networks which did not connect to or interact with the Internet. The Internet, and it's protocols (IPv4 and IPv6), have essentially "won"; it's incredibly rare to find a network that doesn't support them anymore. Even more so for a network using Active Directory.
I found this as a possible explanation in the technet article:
Also, there exists the possibility that you can't have all the RPC ports open between sites (135 + high ports). Just using port 25 may be your only option.
I wonder if anyone uses this to get through a firewall. I've never liked the idea of opening windows RPC ports through a firewall
Probably unnecessary these days for the reasons given above. Also, if one is concerned about firewall issues, you can force DCs to use static ports.