How do I delete large blocks of emails in iPhone mail application

You can choose to delete ALL at once, and mark several at a time for deletion.

1. Open your Mail app

2. Tap on the mailbox from which you want to delete mails.

3. Tap Edit in the upper right corner

Now you can choose to mark the emails that you want to delete. When you mark one or more emails, you will see a Delete button in the lower right corner. Use it to delete. You can also select "Select All" in the top left corner. This will mark ALL emails, and then you can press Delete as before.