Here are some steps to troubleshoot why your emails are disappearing from your sent folder:

1. Check your Settings: Log in to your account settings and review default settings to make sure your emails are saving in your sent folder.
2. Check your Anti-virus Program: Your anti-virus program may be deleting emails that it detects as suspicious. Temporarily disable the anti-virus program and check if the emails start appearing in your sent folder.
3. Check Multiple Accounts: Check if this issue is apparent in all of your email accounts or only in one account. You may need to troubleshoot both the email provider and the settings on your device.
4. Update and Reset your Application: Check for updates and then reset your email application.
5. Enable IMAP Access: If you are using POP3 access turn off POP3 access and Enable IMAP access.