Teachers can send messages via WebUntis:

In the app: Teachers can send you a message on your cell phone via Messages. There can be a "Read receipt" option. The sender will then immediately see when you have sent the read receipt and have therefore read and acknowledged the message.

Web interface: Letters to parents can be sent by teachers as an attachment via the web interface for messages with read confirmation. Teachers will then immediately see that you have read and acknowledged the message.