There could be any number of reasons you haven't seen the message or a returned email yet. One possibility is that the message is still queued at a server waiting delivery because of some delivery issue (e.g. server timeout, server refusing connection, receiver mailbox full). In such a case it could be up to 24 hours before you get a response indicating it's still queued for delivery.
In order to check on a sent message we would need to know information such as your account username and details of the message (subject, to-address, approximate date/time sent).
Please submit a support ticket at
https://support.runbox.com if you would like us to investigate the sent status of the message.
Regards,
Rich