The most common cause for this is that you have configured an external mail server. When adding a webhosting package, we always configure our mail server to accept mail for this domain (for shared hosting, but also if you have your own server or VPS). It’s possible however that you do not want to use our email server. This doesn’t cause any problems, unless email to this domain passes through our mail server. That happens for example when you have a mail form on your website that sends the data to an email address on this domain.
What happens is that our mail server receives the email and wants to deliver the email on our system, instead of forwarding it to your external mail server. This won’t work, and the mail will bounce. Unfortunately, that bounce can’t be delivered for the same reason, and your email effectively disappeared.
The solution to turn off maildelivery in your Customer Care Center. This can be via Email Settings-> Email Master Switch -> Click Disable. Your domain will be removed from our mail server so that e-mail will be delivered to your mail server.