When emailing either a link from your platform to an Internal User (a user with a log-in), or emailing files outside of your platform to a Public User, these actions will get recorded against the item's Usage details.
To view this information, select Usage next to the relevant Resource, Database Record, News Item, Survey Response, Agenda, or Custom Page.
As well as seeing other Actions, such as Download and View (where applicable), you will also see when emails have been sent out against this item, the date/time when this occurred, and who sent it:
From here you can click on (Show Details) against each Email Action:
If this was a Public Email to a Resource then you will see the email addresses to whom it was sent:
If an email was sent to Internal Users you will also get an option called Access Tracking:
This will then show you if the users have clicked into the file from your email, and the date/time this occurred.