I had a client receive over ten thousand copies of the same message before I manually removed it from the spool. This is in SmarterMail version 15.1.6005.

Here are relevant entries from our Delivery logs (recipient's email address has been removed)

 

16:29:00 [72630] Delivery started for do-not-reply@yahoo.com at 4:29:00 PM
16:29:06 [72630] Starting local delivery to [removed]
16:29:06 [72630] Exception: An item with the same key has already been added.
   at System.ThrowHelper.ThrowArgumentException(ExceptionResource resource)
   at System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add)
   at RelayServer.DeliveryManager.#BRj(LocalRecipient #w5i, VCalendar #ARj)
   at RelayServer.DeliveryManager.#yRj(LocalRecipient #w5i, String #sUb)
   at RelayServer.DeliveryManager.#SBb(SpoolMessage #Nrb)
16:29:09 [72630] Starting local delivery to [removed]
16:29:09 [72630] Exception: An item with the same key has already been added.
   at System.ThrowHelper.ThrowArgumentException(ExceptionResource resource)
   at System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add)
   at RelayServer.DeliveryManager.#BRj(LocalRecipient #w5i, VCalendar #ARj)
   at RelayServer.DeliveryManager.#yRj(LocalRecipient #w5i, String #sUb)
   at RelayServer.DeliveryManager.#SBb(SpoolMessage #Nrb)
16:29:12 [72630] Starting local delivery to [removed]
16:29:12 [72630] Exception: An item with the same key has already been added.
   at System.ThrowHelper.ThrowArgumentException(ExceptionResource resource)
   at System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add)
   at RelayServer.DeliveryManager.#BRj(LocalRecipient #w5i, VCalendar #ARj)
   at RelayServer.DeliveryManager.#yRj(LocalRecipient #w5i, String #sUb)
   at RelayServer.DeliveryManager.#SBb(SpoolMessage #Nrb)