Yes, server rebooted. MS Outlook seems to have less issue (only 2 customers had this problem).
The worst is EM Client, with dozens users reporting it...
Downgrade immediately solved...
Gabriele Maoret - Head of SysAdmins and CISO 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)