There are a number of things to check in this case:
Check whether the domain name is properly linked to your hosting package. If this is a new domain name, it may last up to 24 hours before e-mail messages can be received.
Your mailbox might be full. In DirectAdmin, you can see this under e-mail accounts. Quota is the storage limit, which you can increase by clicking Change next to the e-mail account. If you set it to 0, all disk space of the hosting account can be used for e-mail.