diff --git a/internal/listmonk/listmonk.go b/internal/listmonk/listmonk.go
index 6325565..8387a14 100644
--- a/internal/listmonk/listmonk.go
+++ b/internal/listmonk/listmonk.go
@@ -336,6 +336,12 @@ func (in CampaignInput) payload() map[string]any {
 		"content_type": "markdown",
 		"body":         in.Body,
 		"type":         in.Type,
+		// listmonk's create/update handlers default an omitted messenger to
+		// "email" before validating, but the test-send handler validates the
+		// raw request body as-is — an omitted messenger there fails with
+		// "Unknown messenger .". eec-campaigns only ever sends email, so set
+		// it explicitly everywhere rather than relying on that asymmetry.
+		"messenger": "email",
 	}
 	if in.FromEmail != "" {
 		p["from_email"] = in.FromEmail