I was a Bellsouth user which is now AT&T. Recently they farmed out their email infrastructure to Yahoo. Here is the issue: I received some mail on my BB, then the following day I get a message saying that I need to verify my account in the Blackberry Internet Service because messages are not being delivered.
I will then go into the admin piece in the BB and key in the password. About 4out of 5 times it will kick back and tell me the password is invalid. Then,
it will all of sudden work, and I will receive messages for a couple of hours,
then it will revert back. And, before you ask, I am dead certain that I am using and typing in the correct password. I am an IT professional with admin authority for 6 servers and 150 users. This ain't my first rodeo.

I am seeing the exact same issue on Outlook 2010, when one minute it will accept my password, then later it will not. And in both cases the password is being stored.

I have experienced the same problem on the webmail function too.
When I call Bellsouth(ATT) support they point the finger at Yahoo.
And of course Yahoo makes it doubly difficult to find a phone number for
someone to talk to.

Does anyone have an inkling about what is happening ?

It's not you, and it's not RIM. It's Yahoo.

Your issue is probably a different event than I experienced a couple weeks ago, but I was prompted to validate my Yahoo address in BIS. BIS said my password was wrong. I only retried once or twice then went to Yahoo on a computer. After several unsuccessful login attempts, Yahoo prompted me to change my password, stating my email credentials may have been compromised. I changed my password in Yahoo and then validated my address in BIS and all has been well since.

I've also had times when either Outlook or the Windows Live email app is unable to connect to the att.net or bellsouth.net inbox for a while, and then mysteriously sort itself out.

All that to say it's Yahoo. I don't know what you can do about it except complain to AT&T and Yahoo.
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I suggest troubleshooting with another email account like Gmail and see if it produces the same result. If not, then you know it is a yahoo issue.

I already receive my work mail on my BB and it's the POP type of email - works just fine. Too fine sometimes. LOL