There are several ways to manually send an email to a guest.
From the Invoice
From the invoice screen, in the Guest Correspondence section...
Click to view this guests entire correspondence history
Click on the Mail icon to send a message, if you choose the icon for a PDF template, that template will be selected automatically
Print any of your PDFs using the Print icon
Click this icon to choose which of your PDFs are displayed as quick send options in this interface.
From the Guest Control Panel
Click the Correspondence tab, then the Send Message option