On the main client area menu, click on the My Emails link, which is the last link.
This takes you to a page called My Emails, which is a complete history of all email messages that DomainRegister has sent to you.
It includes invoices, order confirmations, supports tickets, payment confirmations, and more. Just click on the Message Subject to read the email.
Be sure to check this area often, so that you can see what emails have been sent to you.