How do I manage mail storage on my iPhone XR?

Iphone Xr. Mail storage is at 21GB of 64GB. Only 10GB available and can’t do current update to 18.5. How do I reduce mail storage? Use phone for work and personal, but work is only 150 emails. I don’t have large attachments. Why do I have 21GB of mail?

[Edited by Moderator]