Last summer, my mother-in-law reported that she was unable to send email using Outlook Express. I did a little research and found out that I had to configure Outlook Express on her machine to use a special SMTP server provided by her ISP. At the time, I assumed that her ISP was blocking the traffic to her regular SMTP server.
Did I have it backwards? Is it the SMTP provider blocking traffic from IP addresses associated with her ISP in order to block spammers that are associated with her ISP?