Using Outlook 2002, I have created two mail accounts. One mail account is for work (firstname.lastname@example.org), and the other is for my free Yahoo account (email@example.com).
1.) If I send a message from Outlook 2002 using the free mail account, and I send this message to firstname.lastname@example.org, the e-mail will correctly arrive via Outlook in my inbox. The From line of the e-mail correctly lists email@example.com as the sender.
2.) However, if I reply to this message, Outlook will automatically try to send the reply to firstname.lastname@example.org.
3.) So, I redid step #1, making sure in Outlook to set the "Send replies to:" option to email@example.com. After sending this message, I verified (via Outlook) that the sent message still has the "Send replies to:" option correctly.
4.) Upon receiving the e-mail via Outlook, the behavior is still the same, as replying to this e-mail will attempt to send to firstname.lastname@example.org. Looking at the header information in outlook, it shows that replies should be sent to email@example.com.
I'm guessing the Outlook or YPOPS is not conveying the "Reply to" information correctly to Yahoo free.