Outlook can not establish connection…

Computer running Windows XP with Microsoft Office 2002 SP3 can receive e-mail and browse internet but can not send e-mail. Upon testing account settings, error message “Establish network connection: Unable to connect to the Internet or to the network”.

When sending and receiving the message “Your outgoing (SMTP) e-mail server has reported an internal error. If you continue to receive this msesage, contact tiy server administrator or Internet server prodiver (ISP)”.

Any suggestions? ISP says it is on our end, firewall in place and configured appropriately.