I am having the same issue. I have had my phone for a year and all of a sudden as of yesterday my Yahoo and ATT.Net emails are not pulling. My friend has an INC also and his Hotmail address is not working.
It seems some peoples are working fine. The girl that sits next to me has an INC as well and all is working just fine on hers. I of course compared everything and all accounts are set up the same. Something has to be up. I have been singing this phones praises for almost a year. Hell three people in my office bought an INC because of all my kudos about this phone. If no one has a fix it looks like I need to call big red.
I accidentally erased all my E-mail accounts on my DINC yesterday when I trying to free up phone memory. (another issue) I was able to reset TWO of my E-mail accounts (WIndows Live and my Exchange Server), but no luck with my Yahoo and Centurylink... Still says failed, authentication etc..
I can log into these Accounts via the computer, but my phone will not allow it.