@moggy321 You may want to start a new discussion because your issue doesn't appear to be POP server related, that's for the Incoming connection, you want to look at SMTP or IMAP for Outgoing related problems.
The only time I've had trouble sending was regarding a certificate failure for which I had to "manually" approve, I believe they should get automatically updated either via the host or the system itself, mine doesn't appear to have done that yet.
This discussion should give more info and the dangers of approving unknown certificates.
The only other thought if it's not the above is that some Mail clients when trying to connect cycle through certain settings, so yours might have changed the Port or SSL setting in the attempt to connect to the SMTP server. I'm not up to speed with Windows and Outlook but this may help you find the correct section.
The settings macOS Mail uses with regards to Outgoing (SMTP) are as below.
Hope some of the above helps, as I said at the start, it may still pay you to start a new discussion because if the issue is ongoing and with TalkTalk having "fixed" the last major problem over the weekend, yours could be completely different, just a thought.