Well the reason is listed in your screenshot: bad signal while Mail was active or worked in the background.
Nothing kills the battery more than bad cell reception, as the phone has to put more effort and therefore energy into upholding communication.
As most of your consumption seems to be while the screen is off turn off background refresh for mail to A/B test if power drain goes back to normal levels. Alternatively dial back how often Mail should fetch new mail.
















Personally iOS seldomly betrayed me with it’s judgement regarding battery drain and the possible root cause. 🤷♂️ And there still is a difference between 5G with four signal bars and 5G with one. Not all 5Gs are built the same.
You can choose to ignore iOS analysis, but I think you would do yourself a disservice. That should be the first thing to debunk/confirm from a debugging perspective.