Are Microsoft Support seeing a bounceback error message from the TalkTalk servers? If so it would be good to know the description and the TT three digit error code to be able to know what's happening.
Microsoft will be sending fully authenticated mail so there would be no reason to apply a network block.
You mention turning off a Blocked filter? Has your mailbox got Rules defined including a Blocked Senders filter rule inherited from the email platform prior to the last platform we've just been migrated from? If so, that Blocked Senders rule is probably so out of date and potentially flawed that the recommended action is to delete it. Are there any other Rules defined?
