Our users are sad of saving their user passwords in their crm mailbox, to use ACT-Sync. Every time they change their password, they also have to think of changing the password in CRM. Therefore we tried to use one user defined in the email server profile for synchonization.

Based on the https://docs.microsoft.com/en-us/dynamics365/customer-engagement/admin/best-practices-server-side-synchronization, this is only supported for in- and outgoing sync, not for ACT-Sync.
"Delegation (Use Impersonation = No) is not supported for syncing Appointments, Contacts, and Tasks."

Would be great, to also have this for ACT.