Down event mails are not sent

Troubleshooting
3

Hi Christian,
I think we have the correct notifications rules installed (see attached screenshot).

2020-10-06 11_52_25-Followup down event mails _ https___forum.checkmk.com_t_down-event-mails-are-not
2020-10-06 11_52_25-Followup down event mails _ https___forum.checkmk.com_t_down-event-mails-are-not1015×929 35.8 KB
So I think when an device that is UP goes DOWN, then the email should be sent. The other way around works well (from DOWN to UP).
We’re running Checkmk v1.6.0p11.

What could be the issue ?
Thanks
Joeri