Hello again RatztheVicar,
Thanks for providing clarification on the issue you're experiencing. If you're able to successfully send and receive emails using another device signed into your Yahoo email account, and/or on https://mail.yahoo.com, the issue may be related to the app itself. Another way to help narrow it down to that app is by sending a photo using iOS Mail app and see if you receive it successfully. If you aren't signed into Mail on your iPhone, here are steps on how to do that: Add an email account to your iPhone, iPad, or iPod touch.
I also recommend trying out the steps listed here: If an app unexpectedly quits, stops responding, or won’t open. Although the title of that support article doesn't describe the issue you're experiencing, the steps are still relevant. Give those a try and see if they help.
Cheers!