SoHoServer -- Mailserver can not send outgoing mails!

Hi everyone. I have a problem when send outgoing e-mails to other isp mail-servers such as gmail accounts and yahoo e-mail accounts.My server has a dynamic IP which provided by Yahoo SBC DSL so I use a No-IP Dynamic Update Client to map my dynamic IP to a domain name. The problem I have is that I can only receive emails but not sending out e-mails. I had followed the instruction provided by someone in the fedora community but still doesn't work. Can anyone tell me how to fix "port 25 timeout" problem when trying to send out emails? Thanks!

AndyGreen's instruction from fedora forum
-----------------------------------------------------------------------------
If you are on a blackholed IP

Some mailservers you might want to send mail to may reject your mail purely on the basis of your IP address being a residential cablemodem. They figure that since it is not common for average users to be sending "real mail" direct from their IP, it is likely spam and they simply won't talk to you. If this is the case for you, (you can tell by watching /var/log/maillog where you will see the remote mailserver tell you to go boil your head) you need to get Postfix to relay your outgoing mail through your ISP mailserver, which they will accept mail from. This is easy, add this line to /etc/postfix/main.cf

relayhost = yourisp.com
-----------------------------------------------------------------------------------------------------------------------

Here's the error meesage from mail log:
------------------------------------------------------------
Aug 10 22:54:56 localhost postfix/smtp[31167]: 57C1E47ADB: to=, relay=smtp.sbcglobal.yahoo.com[68.142.198.11], delay=0, status=bounced (host smtp.sbcglobal.yahoo.com[68.142.198.11] said: 530 authentication required - for help go to http://help.yahoo.com/help/us/sbc/d...pop/pop-11.html (in reply to MAIL FROM command))
Aug 10 22:54:56 localhost postfix/cleanup[31156]: EB3ED47A7D: message-id=<20050810225456.EB3ED47A7D@rapidreprographics.myvnc .com>
Aug 10 22:54:56 localhost postfix/qmgr[31079]: EB3ED47A7D: from=<>, size=3473, nrcpt=1 (queue active)
Aug 10 22:54:56 localhost postfix/qmgr[31079]: 57C1E47ADB: removed
Aug 10 22:54:56 localhost postfix/local[31170]: warning: database /etc/aliases.db is older than source file /etc/aliases
Aug 10 22:54:57 localhost postfix/local[31170]: EB3ED47A7D: to=, relay=local, delay=1, status=sent (delivered to maildir)
Aug 10 22:54:57 localhost postfix/qmgr[31079]: EB3ED47A7D: removed
-----------------------------------------------------------------