I would like to thank Laurel for having done some pretty thorough testing. I have the same issues and initially assumed my mail client, Thunderbird, which has served me brilliantly for some years, had made a change over which I had no control. NOT SO!
My experience, and also my wife's, on a different PC, matches Laurel's exactly - using live.com, gmail and another mail account for various test messages, it is the Microsoft server which is now misbehaving.
Looking a bit further I find at http://www.express.co.uk/life-style/science-technology/668443/Windows-10-Hotmail-Live-Mail-2012-Upgrade-Outlook that whilst the newspaper apparently knows "Microsoft is about to retire its Windows Live Mail service and has warned
Hotmail and Outlook.com users to move to the web or choose a modern desktop app......". I (plus wife) have 4 regularly used Live/Hotmail accounts and whilst some Current Affairs type stuff has arrived from Microsoft, absolutely no advice on the mail services
changing significantly has appeared!!
By my reckoning Thunderbird is an up to date piece of software (I had it's latest update today) and it is completely inexcusable for Microsoft to hide behind Javier Soltero's "enhanced performance, security and reliability" in screwing up a fundamental e-mail
privacy facility, the BCC. Surely there are common standards for all e-mail servers and my conclusion has to be that MS has unilaterally decided to change something..... otherwise they would work as they have for years and still should work.
I've just tried using the Web access on a couple of the accounts and discover that the new interface is useless for me (all messages in one long list and abysmal response time to open one, lists of contacts have lost their names when opened, so are listed in
alphabetical order of e-mail address) - and on one account finding mail itself was impossible though I could see all the account info. Next step: "Choose a modern desktop app" - I suppose Office 365 is recommended? - but the post from Ron 31 May suggests that
also fails to communicate as it should. Does anybody know one which does? Thunderbird was great and still should be.
Is Microsoft really trying to drive users like me towards Linux and steer clear of Microsoft anything as far as is possible?
Thanks also for some relevant correspondence at https://bugzilla.mozilla.org/show\_bug.cgi?id=1279558 - which guided me here.