Hello Daemach, typically the system administrator accounts are a username without an @domain.com at the end. Have you tried logging in with daemach instead? Alternatively, please try the process Rose provided to reset the administrator credentials as this should get you going in the right direction. For reference, we have several account types:

System Administrators can access all system level settings, impersonate domains/users, etc.  Domain administrators may modify any/all domain settings, impersonate users on that domain, etc. Usually a system administrator will log in with a standard username whereas domain administrators will log on with a full email address as their username. Once logged on, the System Administrator web interface should be clearly and obviously different from the interface presented to the Domain Administrator, whereas those admins simply have an additional menu item at the top for Domain Settings. I hope this helps!