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?

  • This is a great question, and is something I've wondered frequently. I've never needed this functionality, but I suppose if site-to-site vpn weren't an option, SMTP/TLS site links to a remote site may be a decent alternative. That's really the only use case I can think of. – EEAA Mar 10 '11 at 14:30
  • about half way down in this article it discusses SMTP site links in depth. technet.microsoft.com/library/Cc961785 – BoxerBucks Mar 10 '11 at 14:36
  • 1
    The problem is, by looking at documentation (I've never actually used them, too), it looks like these site links don't use real SMTP mail delivery... they use the SMTP protocol, but they use it with direct TCP connections between domain controllers. – Massimo Mar 10 '11 at 14:37
  • Additionally, from the article, it's only for sync between domains, not within a domain. "Therefore, replication between sites over SMTP is supported for only schema, configuration, and Global Catalog replication" – mfinni Mar 10 '11 at 16:53

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:

SMTP is best used between sites where RPC over IP is not possible. For example, SMTP can be used by companies that have a network backbone that is not based on TCP/IP, such as companies that use an X.400 backbone

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.

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