550 Request failed; Mailbox unavailable error - Microsoft Community
https://answers.microsoft.com/en-us/outlook_com/forum/all/550-request-failed-mailbox-unavailable-error/5b7c4361-1ee9-422c-9de4-3e14fe64f67b
WebMay 22, 2019 · This is due to a bug in the latest release of Outlook 2016. Beginning with the May 2019 release, emails sent from an Outlook account that has a Reply-To are instantly rejected, with a message 550 Request failed; Mailbox unavailable. When the Reply-To is removed, the test email sends correctly.
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