If you create an SMTP Virtual Service (with 25 as the port), the ESP feature is available when you select the Enable ESP checkbox but with a reduced set of options.

Enable ESP

Enable or disable the ESP feature set by selecting or deselecting the Enable ESP checkbox.

Connection Logging

Logging of connections can be enabled or disabled by selecting or deselecting the Connection Logging checkbox.

Permitted Domains

All the permitted domains that are allowed to be received by this Virtual Service must be specified here. For example, if you wish the Virtual Service to receive SMTP traffic from john@progress.com, then the progress.com domain must be specified in this field.