Sky have made some changes to their email system which has broken my email accounts.

They have stopped non-SSL on their own email accounts when you use your own client and at the same time I now can't send emails via my Yahoo! accounts.

Having spent a few hours this evening checking everything over and over and trying different ports I found this post:

Changes to Sky Yahoo! Mail - Sky Community

I tethered two laptops to my phone and the messages went first time.

Has anyone else had any problems sending emails?