In our Exchange 2013 environment we have a few users who have tasks that can occur once every 5-10 years. Unfortunately this doesn’t sit well with our retention policies and they get moved down to the archive mailbox. To disable them from getting archived, create a DWORD Value in HKLM\SYSTEM\CurrentControlSet\Services\MSExchangeMailboxAssistants\Parameters called ELCAssistantCalendarTaskRetentionEnabled and make sure it is set to 0.

Restart the MSExchangeMailboxAssistants service after making this change.