I can see from your post that you're unable to delete emails from your device. I understand the importance of managing emails directly from your phone and want to help. If this account is an IMAP account not setup automatically, please go to : Settings -> Account & password -> Select the concerned email account -> click on the account -> Advanced -> Deleted Mailbox -> confirm the mailbox is properly selected. Once you have verified this, mark the messages you want deleted as unread using these steps:
- Open the Mail app.
- Select your email account.
- Locate the email messages that will not delete.
- Tap on Edit.
- Select each of the messages that will not delete.
- Tap on Mark.
- Select Mark as Unread.
If you delete them from the web and they still appear on your device, then you'll want to remove the account from your Settings app and set it up again. The link below can show you how.
If you can’t send or receive emails on your iPhone, iPad, or iPod touch
Remove your email account and set it up again
- On your computer, sign in to your email provider’s website. Make sure that all of your email is there or make sure that your email is saved somewhere other than your iOS device.
- On your iOS device, go to Settings > Mail > Accounts.
- Tap the email account that you want to remove.
- Tap Delete Account.
- Add your account again.