Outlook XP will not send email

This has been a tough issue. The config – Dell Inspiron Laptop, Windows XP Pro, MS Office XP, TrendMicro SMB Client/Server. Email is POP/SMTP account. Outlook XP will not send email. The Email Account Test Utility does not send either. Outlook Express with the same settings WILL send email. I have used both the ISP and the Hosted Domain account and no luck. I have turned off the AV and Spam Filter. I changed the SMTP port to 587. I have uninstalled Outlook and re-installed Outlook. I unchecked any suspicious Add-ins. No luck! Are there any suggestions to fix this? Thanks, B