Android smartphones gives you a plethora of communication options, including apps specifically for writing and checking your email. The default apps are Gmail, for a Gmail account of course, or Email, for a non-Gmail POP3 or IMAP account. If you can't get your email to leave the outbox and actually send, you should check on a few common causes and settings to get the app working correctly.
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You should first check to see if you have an Internet connection at all. If your device isn't online, your emails remain in the outbox until you reconnect. If you're online via your carrier's cellular signal, look for the cellphone signal icon in the status bar at the top of the screen. The default icons for data signals are "E," "3G" and "4G," and the icons turn green to indicate an active connection; if they're white, your connection isn't active. If you don't have a data connection, you may need to find a better location where you can receive a stronger signal or contact your carrier for help. If you're connected to Wi-Fi, you should look for a green icon shaped somewhat like an upside-down triangle in the status bar. If you want to connect to a Wi-Fi network, press the "Menu" button at the Home screen and select "Settings," "Wireless & Networks," "Wi-Fi Settings." If the email doesn't send after you've ensured that you have an Internet connection, try turning your device off and then back on again to reset it.
Sometimes an email remains stuck in the outbox if you did not have an Internet connection when you originally attempted to send it and then you established an Internet connection later. You can usually resolve this by opening the email again and tapping the "Send" button. With an active Internet connection, this usually sends the email successfully. In the both the Email and Gmail apps you can locate your Outbox by tapping the "Folders" icon at the bottom of the Inbox screen. If using an older version of Android, such as 2.3 Gingerbread, view messages in your outbox by pressing the "Menu" button and tapping "Go to Labels." In the Email app you can view messages in your outbox by pressing the "Menu" button and tapping "Folders." The Email app has an additional feature: you can tap the "Send Outgoing Messages" option in the Outbox to attempt to send every message, or you can tap individual messages and try to resend them manually.
For various reasons, the Gmail or Email app sometimes does not synchronize accounts automatically. This could be due to a minor glitch in your connection or perhaps to the settings you've selected. Regardless, manually synchronizing your account often sends your email, which then moves it from the Outbox to Sent. The sync option does not display if you're viewing an email, so make sure you're viewing your Inbox or Outbox first. In newer Android versions such as 4.3 Jelly Bean and 4.4 KitKat, just tap the "Refresh" icon to re-sync your messages, which then sends any any emails still in the Outbox. If using 2.3 Gingerbread or older, tap "Settings" first, and then select "Refresh."
If your Android's Background Data or Auto-Sync settings are disabled, your emails typically never leave your outbox until you manually sync the app. The Background Data setting controls Auto-Sync, so if that option is off, Auto-Sync is off as well. If either of these are disabled, your emails may not send. Although this could be preferable if you want to minimize data usage on a phone's plan and only sync using Wi-Fi, you can turn this on if you prefer Gmail and other apps to automatically use data. Temporarily turning it on can also correct problems sending mail; you can always turn the option off later. Press the "Menu" button at the Home screen and tap "Settings," "Accounts & Sync" to view the Background Data and Auto-Sync options. After enabling them, try to refresh or resend your messages. Google also provides an interactive online troubleshooter that you can use to help diagnose Sync problems that may arise (link in Resources.)