Hi P2K_,
Welcome to Apple Support Communities. We do have a great resource on deleting emails on your iPhone.
Delete emails on your iPhone, iPad, and iPod touch - Apple Support
How to delete multiple email messages
1. Open Mail and go to your Inbox.
2. Tap Edit in the upper-right corner, then individually select the emails that you want to delete, or tap Select All.
3. Tap Trash or Archive. If you only see Archive, touch and hold Archive to see other options like Trash Selected Messages.
If you have another way you'd like to suggest to delete emails on your iPhone, we like hearing feedback from our users. You can send in a specific feature recommendation for our teams to review here: Product Feedback - Apple
Take care.