If mail in Newton looks out of date or wrong, a full sync downloads it again from your email provider. On iPhone, iPad and Android you can also clear Newton's saved copy of your mail.
Steps
Desktop
Press ⌘K on Mac, or Ctrl+K on Windows and Linux, to open the command bar.
Choose Sync & Data Info.
Under Data Management, click Force Full Sync.
To clear everything Newton keeps on this computer, open the avatar menu and choose Log Out, then sign in again. Your email accounts come back and your mail downloads again.
iPhone, iPad & Android
Open the menu and tap the Settings gear at the bottom.
Tap Sync & data.
Under Data management, tap Full sync, then Start Sync.
If that doesn't help, tap Clear local cache, then Clear Cache. This removes saved emails for every account, and Newton downloads them again on the next sync.
Good to know
None of these delete anything from Gmail, Outlook or your email provider. Your mail stays there.
Downloading everything again can take a few minutes, depending on how much mail you have.
Your settings, signatures, snoozes and scheduled emails are kept with your Newton account, so they aren't lost.
Newton also downloads your mail again after every app update.