If your server device (your Secure Mail server) is offline and a friend tries to send you Secure Mail it will stay in your friend’s OUTBOX until you are both online at the same time.
This is peer-to-peer Secure Mail. Because there is no third party or server, both your server device and your friend’s server device need to be online at the same time to exchange any Secure Mail.
From your friend’s perspective, the Secure Mail will periodically check to see if you are online. The outgoing mail will just wait until you come online.