When you send message to someone, he will receive a red notification in the top bar on the chat icon near to his profile image. The number inside the circle indicated the number of your unread messages.
Also if he/she turned on email notifications he/she will receive an email confirmation.
If he/she has Baaz mobile app he will receive a push notification and will see an update for the discussions in the menu. Also, he/she will be able to see how many messages he/she did not read for each conversation in a red circle next to the conversation.