Recepients dont receive letters (we know it because it our clients connect via WatApp and ask they)

Recepients dont receive letters (we know it because it our clients connect via WatApp and ask they)

We created campanies but our clients dont received letters.  But delivery status  -100%
AI send request about it (but I cant login to help portal with my account)

If I send test campaing to 3 my addresses - not one dont have received letters (no in spam folder, no in inbox)

If I send txt dormat - letter go to address
If I send test html - I dont receive it in over mail

Why?