It looks like the "With Callout" option for Recipient Verification in 17.5 MR3 is broken.
Added a new mailbox on an internal Exchange Server. Internal delivery works, external delivery breaks with the XG bouncing the message with a "550 Address unknown".
Looks like the callout test is never done, as the Exchange Server SMTP Logs don't show a incoming connection from the XG to perform the address verification.
Turn off "With Callout" and the message is delivered successfully.
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