What option should be selected if the Hub Transport or Edge Transport server needs to relay email to an alternate SMTP server?

Selecting the option for "External Relay Domain" is appropriate when the Hub Transport or Edge Transport server needs to relay email to an alternate SMTP server. This choice indicates that the emails are being directed outside the organization's internal network to an external entity. The concept behind this option is related to managing how messages are processed and sent beyond the local exchange server environment.

When configuring email routing, specifically for relaying purposes, it is critical to identify the nature of the destination. An External Relay Domain allows for emails to be relayed to domains that are not part of the organization's own infrastructure, enabling smooth communication and mail delivery to external partners, customers, or service providers.

Other options like Internal Relay Domain suggest routing emails internally, which would not be suitable for relaying to an external SMTP server. The Public Relay and Secure Relay choices do not explicitly address the need for relaying emails to an alternate SMTP server, focusing instead on broader categories that don't specifically emphasize external routing requirements. Therefore, the emphasis on external communications aligns perfectly with the concept of an External Relay Domain.