I am administering 30 some odd email accounts (POP3/SMTP). Several days ago one of the users could not send or receive any emails and got the message:
None of the authentication methods supported by this client are supported by your server.
She uses OL2007.
The next day another account developed the same problem followed several hours later by a fourth.
The following day a user of OL2003 fell as well.
By setting the 2007 outgoing encryption to NONE, some of the users problems disappeared.
On User, we took her off the net work and plugged her into a router connected to a completely different net work and that worked fine. When we went back to the proper network it worked for about 30 minutes...and then died.
Now everyone can email one another but three accounts can't email to the outside world.
I am going nuts here because of the inexplicable variations. You can have two computers with the same settings and one will work and not the other.

Go to the services -- e.g., Yahoo, Google, etc -- then each's Help to see its services utilizes Outlook