Por padrão, somente um OOO Reply é enviado para cada remetente (não para cada mensagem).
Microsoft
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.