agenda invite notification keeps on coming back

Hello, I received an Outlook agenda invitation. From my Iphone I then added another person (I invited that person for the same meeting, although I did not owne the topic).

Since I keep on receiving a notification.

It says: "Agenda invite - your answer to the invitation can not be sent"

I have already asked the originator to delete the meeting form his calendar but still this message keeps on coming in every 5-10 minutes. Does someone know how to stop this?