Dear All ,  

i have one exchange server 2013 and i lost the power for about 12 hours and after server getting up i found that emails not received at the time the exchange was down and email not their ?

any idea is this correct or if there is a way to get the emails during the time server is down ?

Hi 

It depends on the MessageExpirationTimeOut  setting on the sender server who is trying to send you emails 

By default the value is  2 days . Most likely this value might not be altered .

For further reference refer 

https://technet.microsoft.com/en-us/library/jj891103(v=exchg.150).aspx

If for any reason connectivity to your Exchange goes down power outage or loss of internet or any other reason mail that is sent to your domain will not make it to your Exchange Server.  As Sathish said, it will get NDR'd back to the sender, and the sender may or may not attempt to resend the message.  One thing you can do to avoid this behavior is to purchase a hosted 3rd party hygiene/continuity provider.  Your MX records would point to their servers (which are highly availiable) and will get delivered there first and the hygiene provider will try and route it to your Exchange Servers.  Most of the clients I have worked with use Mimecast, but MessageLabs and Postini are examples of other companies that provide these services.