Outgoing Email

I am running XP Pro SP2 Outlook Express 6.0
My problem is that whenever i send mail it OEshows it to be sent but the recipents are not recieving it (unless I use web based mail whether it be from my ISP or third party like Gmail)I can recieve mail with out problems , although my ISP has recently installed spam filtering ( adjusted with no change to my problem)I have disabled my antivirus but did not effect my problem. I am at a loss where to go next, any help would be greatly appreciated.