THX Robert, It was the spam level of the message and the corresponding Forwarding Exclusion (Domains > Configuration > Email)
Solved
Gabriele Maoret - Head of SysAdmins at SERSIS
Currently manages 6 SmarterMail installations (1 in the cloud for SERSIS which provides services to a few hundred third-party email domains + 5 on-premise for customers who prefer to have their mail server in-house)