We have a really weird situation. One of the email accounts received an email that "A reply for your event has been received".
Now, the problem is that this exact email keeps appearing as a new email every two seconds or so. It appears as a new email, so list of new emails is growing massively. These emails can only be manually deleted. System reports in smtp do not show any activity. Mail content filtering is not working.
P.S. message content: https://pastebin.com/GL0m4ZdQ
This is what I have in the delivery logs:
[2017.08.08] 16:50:08  Starting local delivery to email@example.com
[2017.08.08] 16:50:08  Exception: An item with the same key has already been added.
[2017.08.08] at System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add)
[2017.08.08] at RelayServer.DeliveryManager.#BRj(LocalRecipient #w5i, VCalendar #ARj)
[2017.08.08] at RelayServer.DeliveryManager.#yRj(LocalRecipient #w5i, String #sUb)
[2017.08.08] at RelayServer.DeliveryManager.#SBb(SpoolMessage #Nrb)
Removing the message from the Spool, solved the problem. Interestingly, even though there is this error, emails kept re-appearing in the email.