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
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:
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.