In the event of a power outage, your Helm will safely shut itself down. An on-board battery provides the device with enough power to ensure that your data is secure. Once power is restored, Helm will turn itself back on and resume normal operation.
While offline, emails sent to you will not be delivered. This does not, however, mean that they are lost. If your email server becomes unreachable for any reason, the sender's email server will periodically retry sending the email at a later time. Email servers are generally configured to retry rapidly at first and then back off and increase the time between retries.
Once your email server comes back online and a retry is performed, you will successfully receive the sent email. If your email server is offline for an extended period of time, the sending server will eventually give up and send a bounce message to the person who sent you the email.