> "oops, try again later" is not what is happening when your mail server reboots. Email uses asynchronous protocols.

Wha...? How do you think my server receives emails while it's rebooting?

The sending server will automatically try to deliver the email again for a few days. If the receiving server is occasionally down, there will be a delay, but if it's not down for very long (many days) the email will eventually be delivered.