I have noticed that the error below is happening in most of the emails we are sending to Microsoft accounts:
Email FALIED to send. Reason USER UNKNOWN. Text: 518.104.22.168. Requested action not taken: mailbox unavailable.
Do you know what are the reasons why a mailbox would be unavailable? Is there any way to fix this?
Whomever you are trying to send an email delivery to, that email does not exist under that particular domain, hence;
[REQUEST] Adobe ---- send email to firstname.lastname@example.org
[RESPONSE] Server at someone@someone USER IS UNKNOWN
Adobe then places this recipient under quarantine.
More info --> Adobe Campaign Help | Managing quarantines