We have a hybrid Exchange 2016/O365 implementation. Whenever a user with a milbox in Exchange Online sets their "Automated Replies" a.k.a. Out Of Office the resulting OOF message end up in the senders Junk Mail folder. This only happens for senders from outside the organization. I've tested this with personal mailboxes in outlook.com and GMail with the same results. I can't figure out why they would classify these OOF notifications as junk.
Anyone had this problem and if so did you fix it, and if so how ?
Could you share the header so the I can analyse what is exactly causing the issue. If you can't obviously for privacy reasons try analysing the message header on your end. You could go to https://testconnectivity.microsoft.com/ and analyse message header there.