Recently, my Gmail failed to access my hotmail via POP3:
Server denied POP3 access for the given username and password. Please contact Microsoft's Support Team for more information on POP access. Server returned error: "Authentication failure: unknown user name or bad password. [Error="ProxyNotAuthenticated" AuthResult=0 Proxy=KL1PR0201MB1655.apcprd02.prod.outlook.com:1995:SSL]"
It was working fine for a few years. Suddenly stopped working. I searched and found the reason: Microsoft turned off POP3 access on my account without any notification. So turning it back on solved the problem:
For more details, see here