A catch-all email address receives any mail (sent to your domain name) that isn't addressed to a mailbox. Catch-all mailboxes do tend to attract spam but they are useful if you need to access mail sent to deactivated accounts. To add a catch-all mailbox to your account, log in to the control panel (https://control.gridhost.co.uk) and select the website/domain that you'd like to add the mailbox to.
Click the Email icon and then click the icon to create a forwarder.
The forwarder should be *@yourdomain.co.uk (the asterisk is a wildcard) > email@example.com
This will turn firstname.lastname@example.org into a catch-all mailbox.