There is both a domain-level and a user-level safelist. Domain administrators can add senders to the safelist, and it takes effect for all users in the domain. Users can create their own personal safelist, and it takes effect just for their account. Both the domain's safelist and the user's safelist work in conjunction with each other to ensure that mail from trusted senders always gets delivered. Customizable blacklists work in a similar fashion to block mail.
For customers desiring extreme spam protection, the Exclusive filtering level can be used to block all email from senders not appearing on their safelist. This will cause email from all unknown senders to be rejected by the SMTP server.