It appears the Marketo connector does not make the emails object available as a source. The are listed in the assets api according to Marketo documentation (https://developers.marketo.com/rest-api/endpoint-reference/asset-endpoint-reference/#!/Emails/getEmailUsingGET). This enhancement will allow for the MAILING_ID as the PRIMARY_ATTRIBUTE_VALUE_ID for some ACTIVITY_TYPES to be associated to the mailing.