Which feature of the Cisco Hybrid Email Security services enables you to create multiple email senders on a single Cisco ESA?

http://www.cisco.com/c/en/us/support/docs/security/email-security-appliance/118192-technote-esa-00.html
Virtual Gateways can be used to improve delivery. You can create a group of IP interfaces and use that Virtual Gateway to deliver messages. The ESA will rotate through the IP addresses and distribute delivery over the group of IP addresses. his is advantageous when delivering high volumes of mail to systems that limit connections, messages, or recipients on a per-IP basis.