Welcome to Android Central! What's the exact model name/number? "7" is the screen size, but is it the original Galaxy Tab, or the Tab 2, Tab 3, or Tab 4?
Does this happen on any wi-fi network, or just your home network? Which email app and email provider are you using? If you're using the stock Email app, go to its settings, select your email account, and remove it, then add it back again. If the problem persists, you may have to check the server settings to make sure they match what your email provider uses (specifically, the SMTP server settings).