Exchange 2003 – Greylisting and Delayed NDR’s

Recently I’ve heard from various different clients having problems when sending emails to the NHS The error message usually reads <****.com #4.0.0 smtp;451 ******@****.nhs.uk>… Requested mail action not taken: mailbox unavailable   Due to the message specifically saying “mailbox unavailable” and it’s several different clients all sending to the same organisation I’ve been placing the

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