Somewhat obsolete! This template used to be needed to simplify the inclusion of email links in wiki pages. Now you can simply use a property of Type:Email such as property:Email, e.g.
On pages that belong to a person, organisation, or similar "agent", you should use Template:Mailbox or Template:Person to get additional FOAF annotation in one step,
If you prefer using this template over a property, just write
Doing so creates a semantic annotation for the email, so that you can search for emails later on.