Weird Thunderbird and Win 10 issue - help please

Having upgraded one of my systems to Windows 10, I was unable to get Live Mail working despite trying all the suggestions on the MS and other sites.

So I installed Thunderbird which appears to work OK and have set up severa lcurrent family Hotmail email accounts in it but I am having one very weird problem.

If I send Emails to the Hotmail address's from my XP system using Outlook Express and my normal use email address's the test emails never arrive, all the sent emails are sitting in my sent box with no reports of errors.  If I use the Win10 system and Thunderbird to send emails from all the Hotmail Email address's to my Outlook Express addresses they arrive and if I reply to them then they all arrive correctly back in Thunderbird.

The Thunderbird hotmail email address's do appear to be receiving Emails from the commercial sites that use these address's to contact us.

Before I start using Thunderbird in earnest I really need to understand what the hell is going on and how to fix it.

Any ideas or suggestions much appreciated