1. All techicians receive a notification that a new request has been created (this works OK)
2. All techicians belonging to a group receive a notification, when the request is assigned to that group (this works for all, but one technician)
3. A technician receives a notification when a new request has been assigned to him (this works OK)

Ok, so in a working scenario one tech receives 3 emails. But for some reason one technician receives only 2 emails in similar case, because the notification 2 (assigned to a group) is not delivered. This problematic user has exactly the same rights on SD, and belongs to the same groups and and Sites. This problematic user has been created later than the other 3 techs.

Where to start troubleshooting?

Below are example pictures of working and non-working email notifications.

Working (all 3 messages delivered):

Non-working (group message is missing):