What role in Exchange Server 2010 serves as a secure SMTP gateway?

The Edge Transport server role is specifically designed to act as a secure SMTP gateway in Exchange Server 2010. It handles all incoming and outgoing emails to and from the organization. By placing this role in a perimeter network or DMZ, it adds a layer of security, isolating the mail flow from the internal network and protecting the organization's Exchange environment from potential threats.

The Edge Transport server is primarily responsible for anti-spam and anti-malware protection, which is crucial for filtering out unwanted or harmful emails before they reach the internal system. Additionally, it manages email routing and can also be configured to work with other security measures, adding robustness to the organization's email security posture.

In contrast, the Client Access server role is mainly focused on providing user access to mailboxes, while the Hub Transport server role is responsible for message routing within the Exchange organization but does not serve as a perimeter security layer like the Edge Transport server. The Mailbox server role stores the user mailboxes and is crucial for retaining the emails, but it should not directly handle external SMTP traffic under normal configurations. Thus, the functionalities of the Edge Transport server role collectively solidify its position as a secure SMTP gateway within Exchange Server 2010.