Sent emails from Gmail used to appear in the Inbox of the iPhone Mail app if you enabled a specific setting. Modern versions of Mail do not have this option, and Gmail processes emails more efficiently. If your sent emails are appearing in your Inbox and you don't plan on accessing your Gmail account from another mobile device or email client, prevent Gmail from sending them by switching from POP to IMAP.
How do I fix the issue of sent emails not showing up in Sent folder on iPhone?
Follow these steps if your iPhone is not updated to the latest iOS version. You may have the version of Mail that is experiencing this issue.
Open the Settings app from your iPhone's home screen.
Go to Email > Accounts.
