What email domain are you sending to? Domain is the part of the recipient's email address after the @ symbol.
For example the Gmail domain is gmail.com and we know that some TalkTalk mail messages sent from TalkTalk Mail webmail are arriving in recipients' Spam folders. In those examples the mail is delivered so there's no Mail Delivery Notification of failure to deliver and it's just that Gmail is dropping the messages in to Spam folders.
