Hi JonasGrumby,
For help with ensuring emails are and stay deleted, you can check the time interval for keeping deleted emails with the steps below found in the article here: Delete and recover emails in Mail on iPhone - Apple Support
"Decide how long to keep deleted emails
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, tap iCloud Mail, then tap iCloud Mail Settings.
- If you’re using another email account: Tap your email provider, then tap your email account.
3. Tap Advanced, then tap Remove.
4. Select a time interval."
You can also test on a different Wi-Fi connection and try normal restart of your iPhone. Restart your iPhone
If the issue remains, test the steps here for removing and adding the account back to test: If you can’t send email on your iPhone or iPad - Apple Support
Cheers.