I already posted this over in the WLE>WLM section. I'm posting a copy of it here since the problem appears to have been caused by using the Win10 Tech Preview built in app to access my Hotmail.

Unable to send or receive messages for the Hotmail (sqrly1) account.
Server Error: 0x80048820
Server: 'https://mail.services.live.com/DeltaSync_v2.0.0/Sync.aspx'
Windows Live Mail Error ID: 0x80048820

This started after the 1st time I opened the mail app with my WLID in the windows 10 technical preview 2 days ago.

I can access my mail via a browser.

I can access my mail via the mail app in win 10

I can still use skype with the same WLID

I can no longer access my mail on either of my 2 accounts (the 2nd never used with win 10) with Windows Live Mail on Win7x64 pro.

note: My WLID is the same as my Sqrly1 Hotmail account.