Hi Châu,
Uyền vừa mới tìm thấy cái này trong yahoo.com . Châu làm theo chỉ dẩn step by step, sau này Châu sẽ dùng Outlook Express để đọc và gửi thư chứ không cần sign in vào yahoo, nhớ check vô box "My Server Requors Authentication". Làm xong rồi dùng Outlook Express gửi thư cho Thắng và Uyền thử xem sao. Good Luck.
Uyền

Outlook Express (IE 5) Outlook Express allows you to add a new email account to your existing profile. This means you do not have to replace your current settings in order to send and receive Yahoo! Mail messages. Here's how:

From the Tools menu, choose "Accounts."
Select the "Mail" tab.
Click the "Add" button.
From the Add menu, click "Mail."
In the text box labeled Display Name, type your name and click "Next."
In the Email Address box, type your Yahoo! Mail address (be sure to include "@yahoo.com") and click "Next."
Under "My incoming mail server is a..." select "POP3."
Type "pop.mail.yahoo.com" in the Incoming Mail (POP3, IMAP, or HTTP) Server box.
Type "smtp.mail.yahoo.com" in the Outgoing Mail (SMTP) Server box.
Click "Next."
In the Account Name box, type your Yahoo! Mail ID (your email address without the "@yahoo.com").
In the Password box, type your Yahoo! Mail password.
If you want Outlook Express to remember your password, check the "Remember password" box.
Do not check the boxes labeled "Log on using Secure..."
Click "Next."
Click "Finish."
Important: The Yahoo! Mail SMTP server now requires authentication. To turn this setting on:

From the Tools menu, choose "Accounts."
Select the "Mail" tab.
Double-click the account labeled "pop.mail.yahoo.com."
Select the "Servers" tab.
Check the box next to "My Server Requires Authentication."
Click "OK."
To control deletion of messages from the Yahoo! Mail Server:

From the Tools menu, choose "Accounts."
Select the "Mail" tab.
Double-click the account labeled "pop.mail.yahoo.com."
Select the "Advanced" tab.
In the Delivery section at the bottom of the window, check "Leave a copy of messages on server" if you want to save your Yahoo! Mail messages on the Yahoo! Mail server as well as on your local computer. Do not check this box if you want your messages to be deleted from the Yahoo! Mail server once you have received them in Outlook Express.