Microsoft will soon be making some security changes to their first party email accounts (Outlook, Hotmail and Live addresses). These changes may impact anyone that uses 3rd party apps to view their email on these accounts, or old versions of Microsoft's own apps.
To briefly go into the technical details of what is going to happen here, Microsoft is going to be removing what is known as "Basic Authentication". This is a less secure method of sending your username and password over that they've been moving away from for a few years, and this change will remove it entirely.
If you already use the latest version of Outlook, Apple Mail, Thunderbird and some other modern apps that support Microsoft accounts, you should already be fine. This also goes for connecting to your account from the Outlook / Hotmail website. If you're running an old version of any of these, you'll want to update in the next few weeks. If you're running an old app that hasn't and won't be updated to support the newer connection method, you will find that it will stop being able to send and receive emails from these accounts.
This change will start rolling out from September 16th, so you have a few weeks to get everything up to date in advance of this. If you're using the inbuilt Windows "Mail" app, now would be a good time to shift over to the new Outlook app, as Microsoft will soon be forcing this change anyway.
Microsoft will also be removing support for syncing Gmail addresses within the main browser-based Outlook webmail app pretty much now. This wasn't a widely used feature, but if you're one of the few that used it and have noticed that account disappear, it's gone and it won't be coming back!