fix: wrong data key

runnable-mail.go

@@ -26,7 +26,7 @@ func NewMailRunnableFromMap(data map[string]interface{}) (*MailRunnable, error)
 		return nil, fmt.Errorf("%w: Invalid Subject: %v", ErrInvalidData, data["subject"])
 	}
 
-	body, ok := data["l"].(string)
+	body, ok := data["body"].(string)
 	if !ok {
 		return nil, fmt.Errorf("%w: Invalid Body: %v", ErrInvalidData, data["body"])
 	}