Thank you for replying. Just a follow up question. For offers to work on transactional emails, I should "Modify the options of the targeting dimension" for Real Time Events delivery mapping ?I want to map the offers based on email. Is there anything else I need to configure? 

The message center document says it's possible. But, enough information is not available. https://docs.campaign.adobe.com/doc/AC6.1/en/MCE_Introduction_Interaction.htmlDocument says:The @externalId attribute must contain the value of this identifier. Interaction is configured by default to identify ...