I have seen an issue with Out of Office emails are not working for External user, after checking few things I found that auto-reply has been set to false on my exchange, to check auto-reply setting and enable auto-reply please use below commands.
Get-RemoteDomain Default | fl AllowedOOFType, AutoReplyEnabled, AutoForwardEnabled
If AutoReplyEnabled is false, use below command to enable it.
Set-RemoteDomain -Identity Default -AutoReplyEnabled $true.
When OOF is enabled, all reply emails are sent using Return-Path:<> (=empty), when your email gateway/smart host detects this they may drop the email so the external sender is not notified that you are out of office.
There is no resolution to this on the Exchange 2013 side of things and you must contact your smart host provider to see if they can disable this “feature”.
To check if you are running a smart host, in the Exchange 2013 EAC browse to mail flow then select your Send Connector and then edit. you will then see the smart host option.
To workaround this issue I have created an automatic reply using a template works without any problem, Please find the below steps to create an automatic reply on outlook.
Please follow the below steps:
Step 1: Open outlook – File – Info – Automatic Replies
Step 2: Select Automatic Replies and select send automatic relies and then select rules at the bottom.
Step 3: Select Rules and click Add Rules
Step 4: Select Add Rule and tick the check box Reply with and click Template.
Step 5: Create a template with your out of office message and leave blank in from and to fields.
I found this solution working for me with out any further changes on smart host.