public void resetMailTransaction() {
        this.endMessageHandler();
        this.messageHandler = null;
        this.recipientCount = 0;
        this.singleRecipient = null;
    }
Stops the mail transaction if it in progress and resets all state related to
mail transactions.
<p>
Note: Some state is associated with each particular message (senders,
recipients, the message handler).<br>
Some state is not; seeing hello, TLS, authentication.
src/main/java/de/gessnerfl/fakesmtp/smtp/server/Session.java