You are using a DOMAIN USER (WITH DOMAIN ADMIN permissions)...
To configure that settings you need to login with a SYSTEM ADMIN user.
DOMAIN ADMINS are not enabled to change system settings...
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)