Took me ages and endless calls with BT to discover this so here is a big tip if you use Windows Mail and want it to still work with the 365 upgrade.
If you "modify" the "Account" you have been using prior to the upgrade, incoming mail will work with the new settings but outgoing mail may not.
Instead "Add" a new "Account" with the new settings.
So simple.
Of course, there may be somethiing odd in my registry causing this issue, but in case anyone else hits it, I thought I'd post this.
