said by sdonelan :

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For most users, removing the port 25 filter should be the last choice. If you can't use the AT&T Yahoo SMTP server then here are your options:

1. Contact your mail hosting provider for information about using Port 587 for sending authenticated outgoing email. Since December 1998, Port 587 has been the Internet standard (RFC2476) port for authenticated email submission for mail clients and is supported by most major mail hosting providers.

2. Or contact your mail hosting provider for information about using other alternative access methods to connect to their mail server, such as VPN, Web mail or Outlook Web Access to send outgoing email.

3. Or change your email software to send outgoing mail messages through AT&T (SMTP) email servers, The AT&T (SMTP) server requires authentication using the same Master Account Member ID and password used for the POP server and PPPoE login, you are allowed to use other email domains for your email address.

Change only your Outgoing mail (SMTP) to use AT&T email servers.
Do not change your Incoming email server (POP3/IMAP).
An example of SMTP authentication settings

4. Or if none of the proceeding alternatives work, request removal of the Port 25 filter. Use the Abuse of Service Form to contact the AT&T Abuse team and choose the **Opt Out Port 25 under Abuse Type. Enter your Master Account Member id (also used for PPPoE Login) to opt-out. You don't need to enter your email address at the other mail hosting provider; the filter is based on your login ID only. After you receive a confirmation email to the Master Account Member ID (this sometimes takes 24-48 hours), you will need to logoff the network and turn off your DSL router/modem for 30 seconds to remove the filter.
An example of the Abuse Form

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d_l advises:

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Also as an alternative to port 587, some third-party email providers use port 465, the "designated" Secure SMTP port. AT&T Worldnet and AT&T Yahoo are examples.

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NOTE:
To use Option 3 your AT&T Yahoo! DSL email must work. If you did not complete the Yahoo portion of AT&T Yahoo! DSL registration, your email may not be setup. If you can log on to www.sbc.yahoo.com please ignore this note.
If you cannot log on to att.yahoo.com please read this FAQ
How do I register a username and password for my new account?
Port 25 is not blocked; anything outbound on port 25 that is not destined for a AT&T Yahoo mail server is not routed past the Redback.