Try the following:
Revert everything back to it's original form, then proceed with the following.
This is the SMTP-AUTH clear text method:
Postfix needs to know how to behave as a SASL client. Add the following lines anywhere you'd like in your /etc/postfix/main.cf
relayhost = smtp.ameritech.yahoo.com
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/saslpw
If you already have those lines, then you're set, if not, add them. Make sure you don't have any duplicates.
Create a file called /etc/postfix/saslpw and add the following, using the syntax provided:
The host portion above must match what you used in the "relayhost" line in main.cf. Something as follows:
smtp.ameritech.yahoo.com myid:mypw (your userid and password)
Save the file.
Hash the file.
[firstname.lastname@example.org] postmap /etc/postfix/saslpw
[email@example.com] /etc/init.d/postfix restart
Once you restart Postfix, send a test message through it, and you should see a line in /var/log/mail log similar to:
>, relay=smtp.ameritech.yahoo.com[220.127.116.11], delay=1, status=sent (250 ok 1234567891 qp 12345)