public boolean isMailTransactionInProgress() {
        return this.messageHandler != null;
    }
Returns true if a mail transaction is started, i.e. a MAIL command is
received, and the transaction is not yet completed or aborted. A transaction
is successfully completed after the message content is received and accepted
at the end of the DATA command.
src/main/java/de/gessnerfl/fakesmtp/smtp/server/Session.java