Hellow

I have installed "postfix-1.1.11-11" version as an MTA at my redhat 9 and was able to send mail to @bol-online.com, @hotmail.com and @yahoo.com without configuring anything . But before the installation I removed all the packages of sendmail *.* . From my /var/log/maillog file I can see the following information :-

Jul 23 14:13:50 pc2 postfix/smtpd[3818]: connect from localhost[127.0.0.1]
Jul 23 14:13:50 pc2 postfix/smtpd[3818]: 3DC4890840: client=localhost[127.0.0.1]
Jul 23 14:13:50 pc2 postfix/smtpd[3818]: disconnect from localhost[127.0.0.1]
Jul 23 14:13:50 pc2 postfix/pickup[2385]: 4EEB790844: uid=0 from=<root@pc2.dns2.com>
Jul 23 14:13:50 pc2 postfix/cleanup[3820]: 4EEB790844: message-id=<Pine.LNX.4.44.0407231413060.3817-200000@pc2.dns2.com>
Jul 23 14:13:50 pc2 postfix/nqmgr[2386]: 4EEB790844: from=<root@pc2.dns2.com>, size=2970, nrcpt=3 (queue active)
Jul 23 14:13:57 pc2 postfix/smtp[3822]: 4EEB790844: to=<ekramul@bol-online.com>, relay=mx3.bol-online.com[202.84.36.10], delay=7, status=sent (250 Ok: queued as 2886850856)
Jul 23 14:13:59 pc2 postfix/smtp[3824]: 4EEB790844: to=<romel2010@yahoo.com>, relay=mx2.mail.yahoo.com[67.28.113.11], delay=9, status=sent (250 ok dirdel)
Jul 23 14:17:00 pc2 postfix/smtp[3823]: connect to mx3.hotmail.com[65.54.253.99]: Connection timed out (port 25)
Jul 23 14:17:08 pc2 postfix/smtp[3823]: 4EEB790844: to=<nirapad@hotmail.com>, relay=mx3.hotmail.com[65.54.167.5], delay=198, status=sent (250 <Pine.LNX.4.44.0407231413060.3817-200000@pc2.dns2.com> Queued mail for delivery)

How can I change the connect from "localhost[127.0.0.1]" to "pc2.dns2.com[202.84.35.224]".

You can't. If you're connecting with a local mail client, even if you specify the external address of that machine as your SMTP server in your e-mail client, the network stack is still going to recognize that you're initiating a connection from the local machine to the local machine, and it will send you through the loopback stack instead of putting the packet on the wire.