Postfix botnet spam attack from my server?

[slayer1ss]

i have a centos 5.3 web server with plesk 10.4 everything on the system is fully updated and i am using postfix as a mail server, server is a corporate server with only 2 websites in it... My problem is that 2 days ago i started getting thousands of delivery failed messages and when i checked neither sender or reciever address matched domains in my server, thats when i noticed i was under a botnet attack and someone was trying to relay messages over my server... so i closed relaying, activated parallels premium antivirus, clicked Verify incoming mail at domain keys spam protection and switched on dns blackhole list from zen.spamhaus there are 2 ip address in white list one of them is 127.0.0.0 / 8 and second one is corporate headquarters ip address to allow smtp logging, because of these damn botters my ip address entered at spam list, when i checked from spamhaus they told me that my ip address was a part of botnet attack so i scanned server with root kit hunter and clamaw both returned nothing however i still seem to get thousands of delivery failed messages... i also read this post http://forum.parallels.com/showthread.php?t=209992 there wasnt smtpd.conf files at neither of those locations so i created them... my questions are:

1- since some of our employees are at other locations and use different non-static ip addresses i cant add them to white list and they cant seem to login via smtp which is a must for us... what can i do for this?

2- i am still getting those spam messages, am i missing something is there somethings else that i can do? or did i do everything i can and messages i am getting are just retrys of messages before i took these actions?

3- is it possible for me to configure postfix so that it can only accept outgoing mails from a domain i select?

 

[slayer1ss]

btw i noticed a ton of

Jan 19 04:06:35 userx postfix/qmgr[32192]: 62BDF358558: from=<apache@domainx.com>, size=53508, nrcpt=1 (queue active)

message on maillog eventhough i am not using php's mail function in anywhere... i changed maillog directory on php.ini to track which script is using phpmail but i guess if i disable phpmail from disabled functions those lines would stop am i right? i also added apache@domainx.com to spam blacklist would this cause any problems?