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Shadow redundancy is an Exchange-specific feature (beginning with Exchange 2010) that provides a measure of fault tolerance when relaying email messages with SMTP.

The sending Exchange server sends the EHLO command.

EHLO smtp_sender.example.com

The receiving Exchange server may advertise that it supports shadow redundancy with the XSHADOW keyword.

250-smtp_receiver.example.com
250 XSHADOW

The sending Exchange server can then use the XSHADOW command, thus informing the receiving Exchange server that it plans to utilize this feature.

XSHADOW MGE5N2Q4YjgtNTg4MC00MGYzLWEzNWUtOWE3ZDk4ZGJjMDFlQFVDLUhDMS51bmlib3gubWVA

Question #1 - What is the value of the string that follows the XSHADOW command?

If the receiving Exchange server agrees, it replies with a 250 status code.

250 tStREZcEVUiXW96O4lqrJA==

Question #2 - What is the value of the string that follows the 250?

At a later time, the sending Exchange server will use the XQDISCARD command to query the receiving Exchange server to determine if it was successful in relaying the message.

XQDISCARD 50

Question #3 - What is the value of the string that follows XQDISCARD?

If the receiving Exchange server has successfully relayed the message, then it will reply with a 250 status code.

250 845e8916-2efb-444f-b7ea-5e676ddfa6a5

Question #4 - What is the value of the string that follows the 250?

I've read that it's a unique ID for the message that was relayed, but I don't see where this ID was previously exchanged between the servers.

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+1 for a unique Exchange question with tough to find answers. –  TheCleaner Feb 13 '13 at 15:05

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