There are a few reasons why iPhone mail gets stuck in your Outbox, including issues with the message itself, or with your connection or mail settings. Although it's always useful to know why something doesn't work, sometimes your iPhone just stops doing something basic -- like sending an email -- for no obvious reason. You may never find out why this happens, but throw enough fixes at it, and it'll hopefully start working again.
Check Your Connections
Open Safari to check your cell and/or Wi-Fi network coverage. If you can't open a website, you don't have a connection. This might be just a simple glitch that you can fix by forcing your iPhone to disable its connections and to reconnect to them. Tap "Settings" and switch the slider button on "Airplane Mode" to "on" to shut down all connections. Tap it again to turn it off to re-enable your connections. In Safari, see if you can open a website now. If you can, try to send your email. If your connections still don't work, the problem may be with your service provider and not your iPhone.
Keep Calm and Try Again
Sometimes, networks get busy. Close down Mail, wait a few minutes and open it again. Open "Mailboxes" and then "Outbox." Tap the email and select "Send." If this doesn't work, make sure you typed a valid and accurate email address. Delete the email if it still won't send, create a new one and try to send it. If you're sending an attachment, send a new email without one. If that goes through, the attachment is likely the problem.
Check Email Settings
Your iPhone can't send email if your settings are incorrect -- so even if they've worked in the past, check them again. Tap "Settings" and then "Mail, Contacts, Calendars." Select the email account you've been using and tap its name in the next screen. If you have a record of your settings, check it against those displayed on your phone. If you don't, you can usually find them on your email provider's website, or use Apple's online mail settings tool (see Resources).
Delete Your Email Account
If you can't find an obvious problem with your connection, app or settings, delete your email account from your iPhone and then re-add it. Tap "Settings" and "Mail, Contacts, Calendars." In Accounts, select the email address that's giving you problems and tap "Delete Account." Once deleted, go back into Accounts, select "Add Account" and follow the steps to register the email address again.
Reboot Your IPhone
A restart or reboot may kick your iPhone into gear. To restart, or turn the phone off and on again, hold down the "Sleep/Wake" button and slide the power bar to the right. When the phone is off, hold down the "Sleep/Wake" button to turn it on again. If your email still doesn't send, reboot your system. Hold down both the "Sleep/Wake" and "Home" buttons until you see the Apple logo. Release the buttons and wait for the phone to restart. Neither of these options should affect your saved data, but it's worth backing up your iPhone to iTunes before you restart or reboot in case you experience problems.