Exchange Custom Sender adds two unique capabilities to Exchange Server:
- The ability to send messages from a non-default email address (i.e. use a secondary address as a From address)
- The ability to receive messages at dynamically-created sub-addresses, such as email@example.com
Exchange Server allows multiple email addresses to be defined for each user, and by default, it can receive messages for all of those addresses. However, it is not possible for a user to send a message from a secondary address (as the From address is always set to their primary address), and it is quite difficult for end-users to even see which address an incoming message was actually sent to.
Sub-addressing (sometimes referred to as plus-addressing) enables dynamic sub-addresses to be created. For example, a user with address firstname.lastname@example.org could use the address email@example.com to define the sub-address 'newsletter'. In addition to the plus sub-address separator (+, defined in RFC 5233), hyphen (-) and underscore (_) separators can optionally be enabled for maximum compatibility. Exchange on-premise does not provide any out-of-the-box support for sub-addressing or plus-addressing, although there is limited support in Exchange Online/Office 365. Exchange Custom Sender adds this capability to on-premise Exchange.
There are two separate components of Exchange Custom Sender:
- Server-based Exchange Transport Agents
- Outlook add-in client-side component which is optional but strongly recommended
The additional routing capabilities (both inbound and outbound) are implemented by installing custom Transport Agents into the Exchange message pipeline, which manipulate the from address for outgoing messages, and reroute incoming messages as required. These components must be installed and licensed on every Exchange Server in your organization for proper functionality. In an environment with Edge Transport servers, Exchange Custom Sender must be installed on every Edge Transport server.
The Outlook add-in allows users to select a From address when sending messages. Additionally, it allows the user to see the actual address every incoming message was sent to, irrespective of whether it was processed by Exchange Custom Sender. This is particularly useful when sending messages from secondary addresses, or when using sub-addresses to receive mail.