Email about Private Message is sent even if "Email notification" is turned OFF

Preconditions:
User1 with active email address
User2 with access to forum component

How to reproduce:
1. Login Poulpe as user2, go to Components -> Forum -> Configure
2. Turn OFF email notification
3. Login JCommune as user2
4. Send Private message to user1

5. Check email box as user1

AR:
Email about new PM is received

ER:
Email about new PM is not received (and is not sent)