If a customer reports that they didn't receive a system notification (for example, that a renewal order was due for payment), you can find it in the message log and resend.
It is worthwhile knowing either the name or email address of the user that the message was sent to before performing the steps below, as this is what is used when searching.
To do this, login to your administrator control panel and select Billing from the top right corner.
From the System menu on the left hand side, select Settings.
Now select Message log (within the Operations section)
You can now search for the message by email address, name, subject or date.
Clicking a message allows you to view it, to resend simply select it using the checkbox next to it and click Resend Message from the top menu.