From FAS Virtual Worlds Almanac
E-mail integration is when a virtual world sends personalized e-mails to a user for specific in-world purposes. Examples include sending an e-mail when a user has received a private message or friend request, sold an object, or been invited to join a group or event.
Some virtual worlds send mass e-mails to their users or use e-mail for billing purposes. This is not necessarily e-mail integration.