ahmedsa
Assuming your external MX records are configured appropriately (& based on some of your earlier posts I have doubts about that), then any mail sent to your domain which was undeliverable for an hour, should be sent again by the external mail server after a specified retry time.
Most mail servers will follow the rules & retry periodically (eg every hour or a few) for 7 days, at which time they give up completely & send an undeliverable message to the original sender.
Some mail servers that do not conform to standards only retry for 2 days, then send an undeliverable message to the original sender.
So the "missing" emails should be in a "retry queue" on a mail server somewhere on the Internet, & will be sent to you after a few hours or so, depending on the retry time of that particular sending mail server.
Have patience, everyone thinks email is "instant" & that moments after a message is sent, it should be received, but that is not really the case, as there are many things that can delay email messages for up to 7 days.