Hello Bucket_,
To help resolve the issue with your email storage, you can reduce the amount of emails stored on your iPhone by archiving or moving emails to "On My Mac" mailboxes via your computer or email web account interfaces. Those are not copied to the iPhone.
You can also follow the recommendations provided to free up storage on your iPhone from the article
How to check the storage on your iPhone, iPad, and iPod touch - Apple Support
Use recommendations to optimize storage
In the Storage section of Settings, your device might offer recommendations for optimizing your storage. Tap Show All to see all the recommendations for your device.
Read the description of each recommendation, then tap Enable to turn it on or tap the recommendation to review the contents you can delete.
Best Regards.