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Suppose an organization has multiple Exchange servers. These Exchange servers can use SMTP to relay email to each other. In this case, they would probably use some Exchange-proprietary extensions, such as XEXCH50 and X-LINK2STATE.

Question: Is there another protocol that Exchange servers might use in this case, one that is proprietary to Exchange?

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It's not clear to me what you're asking. XEXCH50 and X-LINK2STATE are Exchange specific SMTP extensions, they are not independent communication protocols. The underlying protocol is still SMTP. What "other protocol" might you be referring to and in what context might it be used? Exchange transfers email using SMTP, whether that's to an external server or to another Exchange server in the organization. – joeqwerty Jan 31 '13 at 16:30
LOL. You've parroted my question back to me, @joeqwerty. Let me try again: Does Exchange ever use any protocol other than SMTP to relay messages to other Exchange servers within an organization? – james.garriss Jan 31 '13 at 17:10
Gotcha. It doesn't that I'm aware of. SMTP is the protocol used to transfer messages, whether they be user emails (internal or external) or system messages between Exchange servers. – joeqwerty Jan 31 '13 at 17:29
up vote 5 down vote accepted

Versions of Exchange newer than Exchange 2000 Server all use SMTP exclusively to transfer messages between servers in the same Exchange Organization. Prior versions of Exchange allowed for proprietary message transfer protocols. There were other protocol options available in 2000 and newer versions of Exchange, though. Exchange 2003 looks to be the last version that supported using X.400 connectors, for example.

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