In the Mail app > Mail, then turn Ask Before Deleting on or off (below Messages).
Tap , then tap Trash Message.
With some email clients, you can set how long deleted emails stay in the Trash mailbox.
Go to Settings > Mail, then tap Accounts.
Do one of the following:
If you’re using an iCloud email account: Tap iCloud, tap iCloud again, then tap iCloud Mail.
If you’re using another email account: Tap your email provider, then tap your email account.
Tap Advanced, then tap Remove.
Select a time interval.
Note: Some email services might override your selection; for example, iCloud doesn’t keep deleted emails longer than 30 days, even if you select Never.