Salesforce Triggered Sends built on a custom object require two specific technical prerequisites beyond the standard point-and-click Triggered Send Definition setup. First, because custom objects are not natively wired into Marketing Cloud Connect ' s out-of-the-box trigger framework the way standard objects like Opportunity or Case are, an Apex Trigger must be created on the custom object to invoke the Marketing Cloud Connect API and initiate the triggered send when a record is created or updated — this is a documented implementation requirement for custom-object triggered sends. Second, the custom object must contain a Lookup field to Lead or Contact, since the Triggered Send needs a resolvable path to a recipient ' s email address; without a direct Lookup relationship, Marketing Cloud Connect cannot determine who the message should go to. A generic hierarchical relationship ' from Contact to Shipment ' (A) is vague and doesn ' t specify the required Lookup field type needed for send resolution. Placement on the Account Related List (C) is a UI/page-layout consideration with no bearing on whether the trigger mechanism or recipient resolution functions correctly. Reference: Marketing Cloud Connect Triggered Sends on Custom Objects.