After accessing the Partner client's user portal, clicking on the "Re-send Fulfillment Email" button a warning appears:
Fulfillment email has been disabled. Re-enable and try again.
The Partner Account from which the client's SSL certificate was requested from had the Fulfillment email option disabled.
To resolve this issue the Partner account administrator will need to enable this option in the account. The steps below enabled/disables that option:
Fulfillment emails can now be sent by the end client from the User Portal: SO16116