When automatic replies (that is, Out of Office) in TechMail are enabled, only one reply is sent to each sender, even if you receive multiple messages from that person.

This feature sends an automatic reply to notify users who send you messages, but your reply is only sent once to each message sender. The count is reset when you toggle the automatic reply setting to "off". Microsoft Exchange Online clears its internal "sent to" list when you disable automatic replies.

There is no resolution needed, as the service is functioning as designed.

If you would like to have a reply sent for every message, use rules instead of the automatic reply feature.