can't connect to att.net on outlook 2016?

Dear @Robert Steffen,

Welcome to Microsoft Q&A Forum!

I’m really sorry for the frustration this has caused. Connection failures that only affect one account (ATT.net) after a reinstall can be disruptive, especially when everything else appears to be set correctly. Based on my research, I want to explain possible reasons why this happens and lay out a clear path to fix it.

The error 0x8004210A indicates Outlook timed out waiting for a response from the receiving POP3/IMAP server. In practice, this may be triggered by a slow or blocked connection to the mail server, incorrect port/encryption settings, large messages waiting on the server, or security software intercepting the mail flow.
Why this often affects ATT.net accounts on older Outlook: ATT.net/Yahoo requires modern authentication (OAuth) or a 16‑character Secure Mail Key when the email app doesn’t support OAuth. Outlook 2016 (now out of support) frequently struggles in this scenario unless the Secure Mail Key is used for both incoming and outgoing server passwords. ATT also mandates specific IMAP/POP and SMTP ports with SSL/TLS.

Important support note Outlook 2016 is out of support (end of support on October 14, 2025). That means no security updates, bug fixes, or new compatibility improvements from Microsoft going forward. For your reference, Microsoft’s official end‑of‑support pages are here: Outlook 2016 and End of support resources for Office – Microsoft 365 Apps (includes Outlook 2016).

Also, because ATT.net is a third‑party/external email service, Microsoft can’t change or troubleshoot their mail servers themselves. We can advise on client configuration, but server‑side issues (authentication, ports, throttling, account keys) must follow ATT’s guidance. However, when I access to ATT website: https://www.att.com/support/article/email-support/KM1010523/?msockid=34ee8451c7bd6a4902a392dec6af6b23 to find the information of configuration from them, the website is blocking me.

Please note that our support is limited to providing guidance based on official Microsoft documentation. Any third-party resources or references shared in my answer are intended solely for informational or consultative purposes. Microsoft does not guarantee the accuracy, reliability, or continued availability of third-party content, and we recommend exercising discretion when referring to such materials. 

So, in this case, I would like to recommend several options that you can consider using them:

• Option 1: Upgrade to Outlook in Microsoft 365 (recommended): This gives you the current, supported Outlook with modern authentication and ongoing security/compatibility updates. Install using Microsoft’s guide: Download, install, or reinstall Microsoft 365 or Office on a PC or Mac

• Option 2: You can try switching to the New Outlook for Windows: You can switch from classic Outlook using the in‑app toggle or download it from Microsoft Store. Guides: Start using new Outlook for Windows

Both options are actively supported and integrate more reliably with providers like ATT/Yahoo that require OAuth or app passwords.

After installing the new version of Outlook, please configure your ATT.net account using the official ATT settings from their website that I mentioned above. If the ATT site is blocked for you like me, you can refer to this alternative guide that I found: https://www.smartlead.ai/blog/att-webmail-smtp-settings

I hope this information can help you to have more insights of this situation. Wish you a pleasant day!

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If you're encountering a timeout error (0x8004210a) when trying to connect to your ATT.net account in Outlook 2016, there are a few troubleshooting steps you can follow:

1. Check Server Settings: Ensure that your incoming and outgoing server settings are correct. For ATT.net, the typical settings are:
   • Incoming Mail Server (POP3): pop.att.yahoo.com (Port 995, SSL required)
   • Outgoing Mail Server (SMTP): smtp.att.yahoo.com (Port 465 or 587, SSL required)
2. Firewall/Antivirus: Sometimes, firewall or antivirus software can block Outlook's connection to the internet. Temporarily disable these to see if it resolves the issue.
3. Internet Connection: Ensure that your internet connection is stable and working properly. You can test this by browsing other websites or sending/receiving emails from another device.
4. Repair Outlook: You can try repairing your Outlook installation. Go to Control Panel > Programs > Programs and Features, select Microsoft Office, and choose Change > Repair.
5. Create a New Profile: If the issue persists, consider creating a new Outlook profile. This can resolve issues related to corrupted profiles.

If you've gone through these steps and are still having trouble, you may want to consult ATT support for any specific settings or issues related to their service.

References:

• Outlook doesn't connect or web services don't work after migration to Microsoft 365
• Connection issues when signing in after updating to Office 2016 build 16.0.7967 on Windows 10