@Override
		public void recipient(final String recipient) throws RejectException {
			boolean addedListener = false;

			for (final MessageListener listener : MessageListenerAdapter.this.listeners) {
				if (listener.accept(this.from, recipient)) {
					this.deliveries.add(new Delivery(listener, recipient));
					addedListener = true;
				}
			}

			if (!addedListener) {
				throw new RejectException(553, "<" + recipient + "> address unknown.");
			}
		}
Class which implements the actual handler interface.
src/main/java/de/gessnerfl/fakesmtp/smtp/server/MessageListenerAdapter.java