Dear All , 

i have currently exchange 2013 single server and we lost the power for about 12 hours , after that i have restart the exchange server and unfortunately the email during this time is not exist !!! 

is this how it should work or there is something wrong ?

Hello

check eventlogs and https://technet.microsoft.com/en-us/library/bb123479.aspx

This is the same response as I mentioned on your other thread:

https://social.technet.microsoft.com/Forums/office/en-US/eefed55b-df55-4d37-82b9-0a9ee0d66049/mail-lost-during-exchange-server-down?forum=exchangesvrsecuremessaging