Overview of SMTP Call-Ahead Recipient Validation
The SMTP call-ahead recipient validation feature queries an external SMTP server before accepting incoming mail for a recipient. Use this feature to validate recipients when you cannot use LDAP Accept or the Recipient Access Table (RAT). For example, suppose you host mail for many mailboxes, each using a separate domain, and your LDAP infrastructure does not allow you to query the LDAP server to validate each recipient. In this case, the Email Security appliance can query the SMTP server and validate the recipient before continuing the SMTP conversation.
You can use SMTP call-ahead recipient validation in order to reduce processing on messages for invalid recipients. Typically, a message for an invalid recipient progresses through the work queue before it can be dropped. Instead, an invalid message can be dropped or bounced during the incoming/receiving part of the email pipeline without requiring additional processing.