Whatever combination of server name (pop.att.yahoo.com or pop.sbcglobal.net for instance) and whatever combination of ports (25, 995 for instance) I cannot make my Windows Live Mail email client to work. The sane is true for any other email client of course. Can you help please. Thank you.

email address you are sending might be not correct or inactive

One problem is that the incoming (POP) mail server shown in the error message appears to be incorrect. The server address should be "pop.att.yahoo.com" (with a period between "att" and "yahoo"). The incorrect mail server address may interfere with authentication of the sender's email address, which is needed before the outgoing message is sent.

The "530 authentication" message indicates your outgoing (SMTP) server likely requires "server authentication". Also, the "Port 25" message indicates your email program is currently trying to send via Port 25, with no SSL; but Ameritech/Yahoo accounts appears to use Port 465 with SSL enabled.

See more on sever settings for "ameritech.net" users:
http://www.dslreports.com/faq/5535
https://motorola-global-portal.custhelp.com/app/answers/detail/a_id/61941/~/i886---at%26t,-bellsouth,-sbcglobal,-ameritech-email-settings

I would suggest you check the settings for your Ameritech email account in your email program. You may need to correct several settings. The steps to access account settings are somewhat different for various email programs (Outlook, Windows Live Mail, Thunderbird, etc.). You should be able to find guidance in the "help" section of your email program.

Yahoo Incoming Mail Server (POP3) - pop.mail.yahoo.com (port 110)

Yahoo Outgoing Mail Server (SMTP) - smtp.mail.yahoo.com (port 25)

I can receive but can't send mail using my POP mail client. Why? If you're able to receive but unable to send mail using your POP mail client (such as Outlook), please review the possible trouble areas below.
SMTP Server
In the fight against unsolicited commercial email (spam), some Internet service providers require their subscribers to send mail through the ISP's own SMTP servers. If you experience problems sending email, please contact your ISP to determine if it requires different SMTP server settings. You will need to use the ISP's SMTP server in place of Yahoo!'s SMTP server when configuring your POP mail client.
The following is a partial list of ISPs that require you to use their SMTP servers. If your ISP is among them, try replacing the Yahoo! SMTP server with the ISP server in your POP mail client settings, or consult the ISP's help files. (Learn more about configuring your client.) If your ISP does not appear in the list below, please contact your ISP directly for more information.
MSN smtp.email.msn.com NetZero smtp.netzero.net Verizon outgoing.verizon.net Earthlink/MindSpring Please visit its help pages AT&T Please visit its help pages Bell Sympatico Please visit its help pages SBC See below SBC Your SMTP settings will vary based on your account setup. Choose the SMTP setting that corresponds to the email address most like the one provided to you by your ISP.
[email protected] smtp.ameritech.yahoo.com [email protected] smtp.flash.yahoo.com [email protected] smtp.nvbell.yahoo.com [email protected] smtp.pacbell.yahoo.com [email protected] smtp.prodigy.yahoo.com [email protected] smtp.sbcglobal.yahoo.com [email protected] smtp.snet.yahoo.com [email protected] smtp.swbell.yahoo.com [email protected] smtp.wans.yahoo.com For more information about AT&T's SMTP requirements, please visit the AT&T help center or contact AT&T DSL support at 877-SBC-DSL5 (877-722-3755) or AT&T DIAL support at 866-SBC-DIAL (866-722-3425).
SMTP Port
If you have adjusted your SMTP server, and you're still experiencing a problem, you may need to use an alternate port. In an attempt to control spam, some Internet service providers now block port 25, which means you may need to use port 587 when sending email via Yahoo!'s SMTP server. Learn how to change ports.
Antivirus Protection
If you have Norton AntiVirus enabled on your computer, you may have to make configuration adjustments to use Outlook or Outlook Express. For more information, visit the Symantec web site.
Authentication
The Yahoo! SMTP server requires authentication. Learn more.

If you looking for POP3 SMTP Mail Servers then it would be
AT&T WorldNet incoming mail server: postoffice.worldnet.att.net
AT&T WorldNet outgoing mail server: mailhost.worldnet.att.net
or
incoming mail (POP3) -> ipostoffice.worldnet.att.net

Just for POP, I think should be somethinnd like
[email protected]
or
pop.att.yahoo.com

Incoming Mail Server (POP3):
Host Name: ipostoffice.worldnet.att.net:995
User Name: [email protected]

Good luck

This means that your user/password combination is wrong for that email server. So double check that you have the right password and user name entered for that account in Outlook. If you're 100% sure you have it right, then you might be using the wrong email/authentication server. If you continue to have problems and are located near Washington DC, feel free to contact me through my website.

the message could not be sent because it rejected as the sender

u need to enable SSL in your email client

What are the incoming pop3 & outgoing SMTP settings.they should be pop.sbcglobal.yahoo.com & smtp.sbcglobal.yahoo.com
Please check

Which antivirus are you using?