The iPad's Mail app lets you pull all of your e-mail accounts together in one application for quick e-mail viewing and response. If you have your Yahoo! mail set up to go through the iPad's Mail app but you find that you cannot send messages, the problem may be in the advanced set-up portion of your account. Services like Yahoo! often use a different server for outgoing mail than for incoming messages, so configuring your outgoing server should solve the problem.
Before you try altering the server information, check to see if the problem is limited to your outgoing Yahoo! mail. Launch your iPad's Safari app or any other Internet browser and search for any keyword you like. If the browser app connects to the Internet, then your connection is not the problem for sending mail. If the browser does not connect to the Internet, check your Wi-Fi or 3G connection. Once an Internet connection is established, try re-sending your Yahoo! e-mail.
Reset Your Account
If you set up your Yahoo! e-mail account on iTunes rather than directly on the iPad, this can sometimes cause glitches. To fix this, you can delete and restore your account directly on the iPad. Launch the Settings app and choose "Mail, Contacts and Calendars." Select your Yahoo! account and follow the on-screen instructions to delete it. After the deletion is completed, return to the mail settings and select "Add a new account." Add your Yahoo! account information and follow the set-up instructions. After the restoration, try sending a test e-mail to see if your account works.
If your Internet connection is fine and restoring your Yahoo! mail did not work, you may need to manually configure your SMTP server settings for outgoing mail. Launch the Settings app and choose "Mail, Contacts and Calendars," then select your Yahoo! account. Choose the right arrow under "Outgoing Mail," where it says "SMTP." On some accounts, you must choose "Advanced" to reach this setting. Tap next to any server that says "Off" to turn it to "On," including the primary and "Other" SMTP servers. If the server address fields are blank, contact Yahoo! to get the addresses for the primary and secondary outgoing mail servers and enter them manually. This should restore your outgoing mail.
Rebooting your iPad may take care of software glitches such as mail not sending. If your account worked previously and you have not changed any settings, a software glitch may be responsible for the lack of sent mail. Press and hold the "Sleep/wake" button until your iPad screen shows the "power off" slider, and then slide your finger over this in the direction indicated. Wait for the iPad to fully power down, then turn it back on. Test your e-mail to see if outgoing mail works.