I have read every single post on this topic (all 8 pages) and have tried every single thing suggested and then some. I started having this problem about 5 weeks ago. I honestly thought I had made an error somewhere, so I jumped through the 7,000 hoops ATT.net has to change your email password. I changed it online and then in Outlook 2007. I continued to have the problem. The box popped up so often, I couldn't get anything done. I would look up from typing only to find I had just typed numerous characters into my password box. I started doing research and tried every thing suggested. I have honestly been trying to solve this stupid problem continuously for 5 WEEKS!!! I finally did a backup of all my Outlook files and folders, uninstalled Outlook 2007 and reinstalled it and then I started completely from scratch. Since I thought I might have a corrupt file in one of my Outlook folders, I waited to put all my contacts and emails back into Outlook. Much to my horror, 10 minutes after setting up the account, the Enter Network Password box popped up again. I needed a new laptop and thought that might be the issue, so I actually bought a brand new Sony VAIO computer and then proceeded to install Outlook 2007 on the new computer. Guess what....The dreaded box was back! I have changed the POP settings from the old to the new inbound & outbound settings. I have tried changing the server port numbers from 995 and 465 to 110 & 25 as suggested above. That totally messed it up to the point I couldn't send or receive in Outlook. I called Microsoft support and talked to 4 different people, before being 'accidentally' disconnected. I went online to Microsoft and tried to do a 'Live Chat' and I was told I might have to pay for support for Microsoft Outlook 2007 since it was an older version and didn't I want to upgrade to Outlook 2010 with all it's many bells and whistles? I sent my husband to Best Buy today and he bought Microsoft Outlook 2010 for about $150. I installed it today on my brand new computer and guess what? The stupid box popped up again! I can't take Outlook 2010 back (nor do I want to, I happen to LOVE Outlook) because I've already installed it and they don't consider this an Outlook or Office issue. They have said numerous times this is an ATT issue. So, we put my husband's work email on, set it up and (it's not an ATT or Yahoo account) and Outlook worked perfectly.
So Dave, from ATT Customer Service, who so rudely told the other customer to go on to his 'free' email carrier or the other Customer Service guys that commented and asked us to check our little boxes or try Thunderbird, no thank you. I need this problem resolved. I was so furious I knew I would blow if I called ATT Customer Service today, so I had my mild mannered husband call. He spent an hour and 43 minutes on the phone, being transferred several different times only to be told we would have to be transferred to an ATT rep and be charged a fee for support since ATT didn't have people trained to help with Outlook issues. This is not an OUTLOOK issue. If you look at the hundreds of different forums, you have to see this is an ATT issue that needs to be fixed. We shouldn't be charged. I spend a fortune on Wireless (4 cell phones), UVerse and Internet and our home phone. We spend right at $570 a month on ATT. I have my ATT email address, the same one for several years now, on all my business cards, and Real Estate signs (those are expensive too!) use it for PayPal and have hundreds of clients who use it to contact me. At first, it was just an annoyance, but now it is down right ridiculous!
Someone needs to actually do some research and send out a fix for this problem. The stupid box has popped up about 15 freakin' times since I started this message. 
Thanks,
Stacy
Oh and by the way, when I tried to reply to this message and had to set up a user account, I tried to use the link sent to me in the email and received error messages. I had the email sent again, used the new link and still received an error message. I then signed out, went to my phone, came in through the ATT support area and signed up with a brand new user name and email address, which I confirmed and miraculously, it worked just fine. It looked like someone might have been trying to stop people from commenting on this particular thread. I wonder why.
)I read the following from another website. However, I am self taught on the computer so reader proceed with caution. All I know is that I did it yesterday with a minor modification and it worked.
Follow the next steps to solve this issue:
1. Close all running programs
2. Click on [Start] » Run
3. Type "Regedit" and click [OK]
4. Open the following folder:
HKEY_CURRENT_USER\Software\Microsoft\Windows\Curre
5. Now choose Edit » New » Expandable String Value
6. Enter "AppData" as a name
7. Double click the new entry
8. Enter "%USERPROFILE%\Application Data" in the value data field.
9. Close the registry editor
I already had "Application Data|Roaming in my registery. I deleted the word "roaming". Then, I clicked the new entry.I had one data pop up screen asking for my password. (This screen is different than the annoying one that ask for my user name and password. i entered the password that I use for AT&T
Today I have had only 1 data pop up screen asking for my password. I entered it one time I have been on the computer for 5 hours. And that is the only pop up I have had all day. Before it popped up every few minutes.
So far so good.
Issue currently going on with "ATT Account Enter Network Password" more like a Yahoo Server Issue since upgrading there Servers. You can have XP, Vista, Windows 7 or a Mac and this message can occur, whether using Outlook Express, Outlook, Windows Mail (Live), Thunderbird or Mac Mail
Your password is usually correct and should not have to be reset either.
Settings for emails programs:
Incoming (POP) inbound.att.net
Outgoing (SMTP) outbound.att.net
(requires authentication)
Incoming mail server: POP3
Incoming/Outgoing User Name: Full AT&T email address, including domain (e.g., test@att.net, test@bellsouth.net)
Incoming mail port #: 995, secure connection (SSL) checked
Outgoing mail port # 465, secure connection (SSL) checked
Previous Settings
Incoming: pop.att.yahoo.com
Outgoing: smtp.att.yahoo.com
Incoming mail port #: 995, secure connection (SSL) checked
Outgoing mail port # 465, secure connection (SSL) checked
Current fix or work around you can use is first go to the www.att.net home page, log into the Member Center. Once logged in, open your email program and your emails should now be able to start sending with no problems. If issue occurs again perform the same steps once again.
You might notice for example if you went to a cafe or other location with internet connection this issue might not come up, compared to have ATT DSL or ATT Uverse service at home.
If you have multiple devices like an Iphone or Ipad they are usually setup for IMAP settings, you can usually setup your Mac Mail as IMAP which can be real time sync with other devices where POP messages can downloaded from server but can have issues with devices that are setup with IMAP.
If using a Mac computer what you might need to do is check that the configurations are correct. Once done you should close out of Mac Mail and make sure to close out from Dock as well. Then log into the www.att.net website, next log into the Member Center, once done you can reopen Mac mail and should have no problems sending emails again.
Again if issue persists make sure to sign back into Member Center, at least until Yahoo Server issue resolved.
