I can't really answer your specific question because I don't use either Windows 10 or Hotmail/Live Mail but I have used Thunderbird on XP and Win7 since it (Tbird) was in the 0. betas.
The commonality I have with you, though, is that all my mail servers, my primary one I won't name, one that came with my mobile phone from Virgin Media and a Google "throwaway" gmail account, all use webmail servers as their platform, each with a lousy interface. So I use Thunderbird (separate account inbox option) to access all three.
When I set it up, there was the option to use IMAP (messages stay on the server) or POP3 (messages download to my PC) and since I'm not always online, I opted for POP3, with automatic deletion from the server when I download. In this configuration, all my replies and new mail goes through an SMTP server at my ISP (none of the above - it does make sense, honest!). While I've not seen your problem, if I did, it would be highly unlikely that Microsoft would be to blame because, a newly composed mail would only run through Thunderbird and my ISP, not through any of the inbound mail servers.
This would not be the case, had I opted for IMAP with the messages on the inbound mailserver, so that might be an issue in your case.
When you said you were using another email, did you mean other than Hotmail or other than Thunderbird?
If your problem is consistent, I'd be inclined to try a different email client (there are lots on the web) to your Hotmail account. If the problem goes away, it suggests Thunderbird might be the culprit, though I think unlikely. If it doesn't, try using the Hotmail send/reply mechanism (I assume it has one) and see if that cures it - if so, it suggests a mismatch between Hotmail IMAP/POP3 and Thunderbird. If you are using Thunderbird IMAP to Hotmail, you could try POP3 (but set it not to clear the messages from the server in case you want to go back). If you try this, you will need to set up an SMTP server for your outbound messages.
If you problem isn't consistently repeatable, that's a whole new bag of worms and much harder to investigate. The above generalities might help but it would take a time to confirm.
I have some serious problems with Hotmail / Live mail accounts and I hope you can help. I flawlessly switched from Windows 7 Pro to Windows 10 Pro with a few glitches that really haven't been an issue. (e.g. I cannot get my Cortana to recognize I'm connected to the net which is how I discovered this new problem, because I only liked Thunderbird anyway.)
I have used Firefox and Thunderbird email applications from the beginning and I like their platform very much, but I recently sent a message to all of my friends using BCC (which is the ONLY way I email multiple individuals) -- however, the entire message was sent regularly, NOT BCC even though my sent file shows it as being sent BCC. One of my friends thanked me for sending him all of my contacts??!!??
I've been reading online that a lot of others have had this problem and there is a controversy as to which is at fault, Hotmail or Thunderbird. Since Microsoft Hotmail is involved I automatically blame them. Over the years I have been well trained by them to know they are wrong 99% of the time.
Regardless, anyone using Hotmail/Live mail with Thunderbird may be in jeopardy of having all of their group addresses revealed to others.
Of course, this doesn't resolve the problem. So what to do now? Have others using Thunderbird in conjunction with Hotmail experienced this? I've started using another email for better security until this is fixed.
I would like to hear your take on this please. Thanks for your time!
--Submitted by PT B.