I know that the 2013 CAS only role server proxies message on port 25. Can the internal server for a subscription be a CAS role only server? or does it need to have the Mailbox role also?

Tom

Hi Tom,

Thank you for your question.

We need to have the mailbox role for Edge subscription. The Edge Subscription process provisions the credentials used to establish a secure LDAP connection between Exchange 2013 Mailbox servers and a subscribed Edge Transport server.

You subscribe an Edge Transport server to the Active Directory site that contains the Mailbox servers responsible for transferring messages to and from your Edge Transport servers. The Edge Subscription process creates an Active Directory site membership affiliation for the Edge Transport server. The site affiliation enables Mailbox servers in the Exchange organization to relay messages to the Edge Transport server for delivery to the Internet without having to configure explicit Send connectors.

The more details could be referred by the following link:

https://technet.microsoft.com/en-us/library/aa997438(v=exchg.150).aspx

If there are any questions regarding this issue, please be free to let me know. 

Best Regard,

Jim

Yeah, this was pretty much what I expected.

Thanks!