Initialize the handler.

        Initialize the instance with the from and to addresses and subject
        line of the email. To specify a non-standard SMTP port, use the
        (host, port) tuple format for the mailhost argument. To specify
        authentication credentials, supply a (username, password) tuple
        for the credentials argument. To specify the use of a secure
        protocol (TLS), pass in a tuple for the secure argument. This will
        only be used when authentication credentials are supplied. The tuple
        will be either an empty tuple, or a single-value tuple with the name
        of a keyfile, or a 2-value tuple with the names of the keyfile and
        certificate file. (This tuple is passed to the `starttls` method).
        A timeout in seconds can be specified for the SMTP connection (the
        default is one second).
        N)