If your iPhone won’t send email, check to see if your internet is on by tapping on Safari and going to any random webpage. It the webpage loads, you will know that you are getting internet and that is not the problem that is keeping your email from sending. If the webpage doesn’t load, then resetting your Wi-Fi connection by going to Settings, Wi-Fi and turning Wi-Fi off and then on should fix this issue.

If you do have internet service, but still can’t send email from your iPhone, go back out on the internet and go to your email provider, such as gmail.com and check to make sure the account is active and that your password is correct. If it is, then restart your iPhone and see if it will send emails. If your iPhone still won’t send emails, delete your email account from your iPhone by going to Settings, Mail, Contacts, Calendars and then reinstall the account.

Source Apple Support