16 June was the date MS started migrating Outlook.com accounts to the sync protocol supported by all MS messaging products.
POP3 and IMAP are still supported.
Two of my three Outlook.com accounts have been migrated. The symptom on my machine was that I could not connect to the server - sometimes, this just requires trying to connect again or entering your password in the dialog.
Expecting some transition issues, I tried the above and then visited Outlook.com.
POP was disabled
If you use POP3 in WLM, you'll need to configure POP3 support again in Outlook.com after your account has been updated.
Settings > Options > POP and IMAP
I don't have any WLM acounts that use IMAP. The only thing I see that has changed is the server address, but I don't think that is a required change right now though it might be in the future if MS consolidates all servers to one server.
Encryption method: SSL
The image shows my Outlook.com POP settings, you might not want to allow apps/devices to delete messages. That setting adds a layer of protection - a delete in a client moves the eMail to a special POP folder, you have to visit Outlook.com to actually delete the mail. It's sort of redundant if you configure the client to NOT delete mail on the server, but .....
Note that the server addresses have changed, the previous pop.live.com and smtp.live.com, ports and secure settings still work though.
The one interesting change is on SMTP - the secure setting is TLS, not SSL.
WLM doesn't seem to have a way to specify TLS, but SSL still works for now.
I also entered the new server addresses (pop-mail.outlook.com & smtp-mail.outlook.com) in my WLM account configuration, but reverted to the previous server addresses (pop3.live.com & smtp.live.com) when that did not initially solve the credentials issue.
The changes I made to WLM were unnecessary and were only to test what works with the new Outlook.com and what doesn't. There might be some lag between enabling POP in outlook.com and WLM being able to connect/authorize, just try again in a while (my 1st account took about 30 minutes, my 2nd account was almost immediate).
On my 2nd account, I simply logged into Outlook.com and enabled POP - I did not make any changes to WLM.
On a side note, it's worth reviewing all settings in the new Outlook.com. I found things that I changed
Manage add-ons: I uninstalled all that I was allowed to uninstall.
Offline settings: ???After you turn on offline access, you can use this computer when it's not connected to a network ???
Great, I was wondering if I could use my computer when it's not connected to a network.
Under Calendar options, there's this oddity
I checked the box and it displayed my time zone. Good for travelers I guess.
Treat the new Outlook.com as a new eMail client - check all of your settings.
Additional inofrmation: Set up an email app with Outlook.com - Outlook.com Help