Do you mean messages sent to this user or sent from this user going to deleted items folder? Sounds like sent to but I have to confirm it...
You have checked the Outlook inbox rule, right? Go to EAC-Mail flow and check the transport rule please.
Run message tracking for the messages and check the "Recipient status" value, be free to post the log here with personal information covered:
get-transportservice|get-messagetrackinglog -messagesubject testtesttest |fl
I am not familiar with Outlook for mac, can he change to OWA for some time and test if the issue would happen again?
Besides, were those messages come from same domain? It's likely that their SPF record not configured well and been deleted by your server.
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