Are you using auto-forwarding from your TalkTalk mailbox to another email service? You say emails are stuck in your TalkTalk Inbox so I assume that what you have set up is an auto forward from TalkTalk but not ticked to keep a copy of the message.
This would suggest that the auto-forward rule has not successfully completed because the redirect could not be done. Have you checked that the final receiving mailbox is not full or near to capacity / mail quota?
It's far more reliable to collect mail messages from a mailbox rather than rely on auto forwarding. Any originating mail service that has set a strict authentication policy of not allowing other mail servers to send on mail messages can find their emails are either not accepted by the destination mailbox or tagged as spam.
Over the last few years more mail services demand fully authenticated mail. Simple auto forwarding will break the authentication of the original message. Techniques to reauthenticate auto forwarded mail have been developed via SRS and ARC. SRS is Sender rewriting service - the forwarding mail service rewrites the email header to authenticate the forwarding servers. But this potentially breaks the DKIM digital signing of the email. So ARC was developed which is Authenticated Received Chain that authenticates the original mail message and forwards it with the authentication results. You're then reliant on the receiving mail service to be able to implement ARC.
TalkTalk does not implement either SRS or ARC. Therefore auto forwarding of received mail that has a strict authentication policy is highly likely to fail anyway. Have you noted the failures to be always from the same originators?
