Mixing Versions of MTAs and Message Stores

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Will a new Messaging Server 6.x MTA function with a 5.2 Message Store?

It depends on your setup. You cannot mix Messaging Store 5.2 store executables with 6.x MTA executables. However, you may deploy a 6.x MTA running on one host delivering mail to a 5.2 Message Store running on another host. In such a deployment, you'll also need a 5.2 MTA on the store machine because LMTP isn't supported in Messaging Server 5.2, but such a setup will function.