James,

From your description, I gather there is a confusion between Modes (Stand-alone, Hybrid, Firewall) and Server types (SMTP Proxy and Regular SMTP). These are two different concepts and you don't have to use the SMTP Proxy server when using Xeams in Hybrid/Firewall mode.

I see you're using the SMTP Proxy server on both Primary and Secondary MX. That is the reason why you get Server Temporarily out of service when the next server is down.

This is what I recommend:

• Don't use SMTP Proxy server. Use the regular SMTP Server. Check https://www.xeams.com/SwitchingProxy2Regular.htm for instructions on how to switch. This is alleviate the dependency of the primary server from the secondary.
• Use Xeams in Firewall mode, not Hybrid. This ensures the User Repository in Xeams does not get filled.

This will meet both of your requirements.

Additionally, I recommend you run Xeams in Clustering mode. What is the reason for not using this feature? This will have additional benefits:

• Both Primary and Second Xeams should forward their emails to Exchange
• If Primary Xeams goes down, emails will go to your Exchange through Secondary Xeams
• You will always be able to search messages from Primary. None of your users should connect to the web interface of the Secondary.