This mailer contains several different transports for your emails. To be sendable, the
emails have to implement SendableEmail, which is the case for emails created with lettre_email.
The following transports are available:
SmtpTransport uses the SMTP protocol to send the message over the network. It is
the preferred way of sending emails.SendmailTransport uses the sendmail command to send messages. It is an alternative to
the SMTP transport.FileTransport creates a file containing the email content to be sent. It can be used
for debugging or if you want to keep all sent emails.StubTransport is useful for debugging, and only prints the content of the email in the
logs.