When it comes to Out of Office messages through the Fortimail there are a couple of challenges to be aware of.
Out of Office message generated by Outlook/Exchange are sent with null sender (MAIL FROM:<>), this can cause issues for both incoming and outgoing notifications through the Fortimail.
First, for incoming OOO messages if you have Bounce Verification turned on the null sender will make the OOO notification look like a bounce and the recipient address will have the BATV (bounce address verification tag) stripped therefore the message will be blocked.
Second, for outgoing OOO from your own mail system those notifications can be blocked by your Access Control policy if you have it configured to only relay if the sender address is “Internal”. You’ll want to make sure that you allow relay for both internal AND null Sender addresses from your mail server. The access control rule shown below will match both null and an internal address for sender and allow relay if the internal IP address matches as well.