Moving encrypt, sign and send logic from gateway (SMTP) to a MailService

Creates a MailService that has the logic for encrypting, signing and sending messages. Factors that logic out of EncryptedMessage so it can be used by other clients
    """
    Initialize the mail service.

    :param from_address: The sender address.
    :type from_address: str
    :param keymanager: A key manager.
    :type keymanager: KeyManager
    :param cert:
    """