Exchange Server 2013 Service Pack 1 saw the return of the Edge Transport role, which was missing in the RTM release.
The Edge Transport role is involved in SMTP communications (email transport), and one or more Edge Transport servers are typically placed in a DMZ to satisfy the needs of organizations who require no direct connectivity between the internal network and the internet. Edge Transport can also serve this role for hybrid deployments with Office 365, so that mail flow between the on-premises organization and the cloud passes through the Edge server.
Edge Transport also contains some additional transport agents that are not installed on Mailbox servers. Here is the complete list of transport agents for Edge Transport:
[PS] C:\>Get-TransportAgent Identity Enabled Priority -------- ------- -------- Connection Filtering Agent True 1 Address Rewriting Inbound Agent True 2 Edge Rule Agent True 3 Content Filter Agent True 4 Sender Id Agent True 5 Sender Filter Agent True 6 Recipient Filter Agent True 7 Protocol Analysis Agent True 8 Attachment Filtering Agent True 9 Address Rewriting Outbound Agent True 10
In comparison, here is the list for the Mailbox server role:
Identity Enabled Priority -------- ------- -------- Transport Rule Agent True 1 Malware Agent True 2 Text Messaging Routing Agent True 3 Text Messaging Delivery Agent True 4
For more on the Edge Transport server role in Exchange Server 2013 see the following articles: