Changing admin password on virtual server: Failed to modify server : Missing or invalid mail username prefix

Hello,

I am getting this error whether logged in as root, reseller, or the actual virtual site admin.

I altered the mailbox name suffix to be that of the domain name
in accordance with this thread.

Why does this need to be done?
<edit>

The field is populated with the administrator user name when it first pops up.

–jason