Mail app draining battery

It depends on the usage. How has your usage changed after iOS update.

Protect & Prolong life of your iPhone's Battery: About the battery usage on your iPhone, iPad, and iPod touch ...

Try some of these options too.

1. Avoid Screen Mirroring
2. Put on Low Power Mode
3. Put Auto Lock On (1 min):- with low power mode it will be 30 Sec
4. Put Auto Answer Off
5. Disable Push notifications for apps you do not use often
6. Enable Auto Brightness:- Settings > General > Accessibility > Display Accommodations > Auto-Brightness [Enable]
7. Manually lower brightness from Control Center
8. If you Want to watch continuous Video. Stream it on TV or a Monitor
9. Don't force close apps; Apps which are not on frontend screen are paused by the system to save energy. Apps require more energy to re-launch. Apps which are open in the background use large memory is a myth and quitting them is counterproductive
10. Location services to be allowed only while using the app
11. Set your iPhone to fetch email and calendar data from internet less frequently
12. Turning off Background App Refresh 
13. Charge before it drops below 20-30%
14. Try connecting to trusted Wifi where ever available in stead of Cellular
15. Enable Airplane mode in areas with bad cellular and wifi coverage. Your phone keeps trying for reconnection with the missing / weak network. This uses lot of energy
16. Switch off Wifi & Bluetooth while in Roaming or Not in Range
17. Enable "Wifi Assist" : Settings > Cellular or Settings > Mobile Data > Wi-Fi Assist [Enable]
18. Periodically Check your Battery Health (Settings > Battery > Battery Health)

iPhone Battery and Performance

You may know that there are two settings (Fetch and Push Mails). What have you set? Watch the GIF below

Google, AOL, Yahoo - Only Fetch mail Option is available

iCloud, Exchange, Outlook - Both Fetch and Push Mail option is available

1. Push Mail: Whenever possible the email client will push mails to all your devices.
2. Fetch Mail: You need to set How you want it to be fetched, Automatically, Manually , Hourly,Every 30 mins, every 15 mins
   1. Automatically: will fetch when the iPhone is powered and on WiFi
   2. Manually: with fetch when you open email, open Inbox and swipe down
   3. You get an email notification as soon as the client server receives an email and your device is connected to internet, irrespective of whether the mail is in your device's Inbox or not

Similar issue on my iPhone 7. Never even setup any accounts, yet draining my battery. Deleted the app and seems to be better.

However, saw another issue with Contacts now. When trying to edit any contact, it will sort of automatically sync/as though I clicked Done. Causing problems with editing contacts and from battery usage, it's contributing quite a bit (like 20%).