Vista - Sometimes I can send email, sometimes I can't

I have been dealing with this for 2 weeks. I have searched this forum, other internet sources, I've been on tech support with my ISP (SBC Global/ ATT) twice for a total of 2 1/2 hours and my webmail provider, I even took my computer to the Geek Squad. No one can solve this problem. I'm about to tear my hair out.

Here's the error message:
The message could not be sent because the server rejected the sender's e-mail address. The sender's e-mail address was 'chiggins@mydomain.com'.
Subject 'here we go again'
Server Error: 553
Server Response: 553 From: address not verified; see AT&T Help
Server: 'smtp.att.yahoo.com'
Windows Live Mail Error ID: 0x800CCC78
Protocol: SMTP
Port: 465
Secure(SSL): Yes


OR this one:
Your server has unexpectedly terminated the connection. Possible causes for this include server problems, network problems, or a long period of inactivity.
Subject 'here we go again'
Server: 'smtp.att.yahoo.com'
Windows Live Mail Error ID: 0x800CCC0F
Protocol: SMTP
Port: 465
Secure(SSL): Yes

I have tried every configuration you can imagine, I've changed ports, I've checked and unchecked "this server requires authentication", and I've completely uninstalled my antivirus software. I finally got it to work for an entire day 2 days ago, and then started getting these errors again last night, then it eventually stopped and my email went through. I've been trying for several hours tonight and it won't fix itself.

I haven't changed any settings in the last 2 days. I have seen many other posts with this problem and haven't found a solution that solves mine - permanently. Please does anyone have any ideas??

Here is my Current configuration (that worked for a day and a half):
Servers: POP3
Incoming: mail.domainit.com
outgoing: smtp.att.yahoo.com

Email user name:
chiggins@mydomain.com
mypassword (remember password checked)
log on using clear text authentication checked
Outgoing Mail Server:
Requires authentication checked.
Settings:
log on using: myaccount@sbcglobal.net (This is my ISP)
password
Advanced:
Outgoing Port 465 (requires authentication checked)
Incoming Port 110 (authentication NOT checked)

Any ideas??? I will be forever in your debt if you can help me solve this. :-)
Thank you!