If you updated your firmware to iOS 11, you may experience this problem. For example, if you used office 365 Account to send Email, you won’t successfully send the email. Usually, the people who updated to iOS 11 experience this.
How to Fix Can’t Send Email Using Microsoft Account on iPhone X / 8 and 8 Plus:
- check your setting first
- check in and outgoing mail server setting
- check username and password are correct or not
- Try downloading the free Microsoft Outlook. It is compatible with iOS 11.
- For the meantime, wait until there is a new version for bug fixes.