A cloud-based service included in Microsoft 365, delivering scalable messaging and collaboration features with simplified management and automatic updates.
Hello @Michal Rama
You can try using Exchange PowerShell like below:
Set-MailboxAutoReplyConfiguration
-Identity ******@domain.com
-AutoReplyState Enabled
-ExternalMessage "External auto-reply message."
-ExternalAudience All
This is the closest supported behavior to sending automatic replies only to external senders.
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.