When the Out of Office Assistant is enabled, only one reply is sent to each sender, even if you receive multiple messages from that person.
The Out of Office Assistant sends an automatic reply to notify users who send you messages that you are away from the office. Your reply is only sent once to a message sender. The count is reset when you toggle the Out of Office Assistant. Microsoft Exchange clears its internal "sent to" list when you disable the Out of Office Assistant.
If you would like to have a reply sent for every message, use Rules instead of the Out of Office Assistant.
Exchange Server 2016 Enterprise Edition, Exchange Server 2016 Standard Edition, Exchange Server 2013 Standard Edition, Exchange Server 2013 Enterprise, Microsoft Exchange Server 2010 Enterprise, Exchange Server 2010 Standard, Microsoft Exchange Online, Office 365