In this case you can look up all your email traffic using our web based control panel.
You can find rejected mails via the "Email Live Tracking" if you select the filteroption "Delivery Status" -> "Rejected":
If you can see the rejected email in the Live Tracking the sender already recieved an automatic reply with intructions on how to ask for these emails being delivered in the future.
You can find more details regarding this topic in the Unblocking Portal.
Using our unblocking contact form is the fastest way to ask for unblocking.