The MMP contains an SMTP proxy that is designed to accept messages but not transfer messages. Because of this design, never use the MMP SMTP Proxy as the target of a DNS MX record or to otherwise receive mail incoming from arbitrary sources on the Internet. Messaging Server does not support the use of the MMP SMTP Proxy in a message transfer capacity.
Messaging Server does support the use of the MMP SMTP proxy for message submission from end-user clients. However, the multiplexing functionality of the MMP, which is necessary to distribute POP and IMAP connections to the correct back-end store, is not necessary for SMTP submission. You can balance SMTP submission by MX records for mail clients that follow the standard, or by a simple load balancer for mail clients that do not follow the standard.
Only use the MMP SMTP Proxy in the following situations:
If the MTA is becoming impeded with SSL/TLS processing, the MMP SMTP proxy can offload that processing for message submission while still supporting standard SMTP STARTTLS.
If the MMP has SSL hardware acceleration for POP/IMAP, it might make sense to also leverage that for SMTP submission.
If you need to use the "POP before SMTP" mechanism, then the MMP SMTP Proxy is required.
The MMP SMTP proxy has a desired feature not present in the back-end MTA.
If your deployment requires a proxy, then use the MMP SMTP proxy, which is specifically designed to preserve the security features and SMTP extensions present in the MTA and uses a custom SMTP extension (XPEHLO) to do so safely.
The MMP SMTP Proxy only works with Messaging Server's SMTP server as a back-end.