One of your inbound servers is in an Ameritech(SBC) /22 block of IP addresses ...
C:\util>nslookup mail.bellsouth.net
Server: homeportal
Address: 192.168.42.1
Non-authoritative answer:
Name: mail.bellsouth.net
Addresses: 204.127.217.17
207.115.11.17
... while outbound is to a Yahoo! block:
C:\util>nslookup smtp.att.yahoo.com
Server: homeportal
Address: 192.168.42.1
Non-authoritative answer:
Name: smtp1.sbc.mail.am0.yahoodns.net
Addresses: 98.138.31.74
98.139.221.42
67.195.15.66
68.142.198.11
Aliases: smtp.att.yahoo.com
smtp-sbc.mail.yahoo.com
An old SBC SMTP server I used to use is in the same Ameritech /22 block:
C:\util>nslookup smtpauth.sbcglobal.net
Server: homeportal
Address: 192.168.42.1
Non-authoritative answer:
Name: smtpauth.sbcglobal.net
Address: 207.115.36.27
I can't guaranty it will work better (or even at all), but give, 'smtpauth.sbcglobal.net' a try. Port 25 should work, but I used to use port 587. Authentication is required; and the user name would be your complete email address.
Definitely answers to Telnet on port 25:
C:\util>telnet smtpauth.sbcglobal.net 25
Connecting To smtpauth.sbcglobal.net...
220 nlpi176.prodigy.net ESMTP Sendmail 8.14.4 smptauth/8.14.4; Tue, 18 Dec 2012
12:48:14 -0600
quit
221 2.0.0 nlpi176.prodigy.net closing connection
Connection to host lost.