I think I tracked this further down. People who are Managers or Developers by default receive mails when bugnotes are added. People who are Viewers by default and Managers or developers only for a particular project don't receive bugnote mails.

We tried to use this as a workaround to keep customers informed of progress. The idea is to create a dummy account with the customer's mail adress, a secret password and manager privileges. This way (we hoped) the customer should receive all mails recarding a certain project.

ok fixed