I don't have any short and easy answers, but do have so tips on what can affect data usage:
- The number of accounts
- The number of folders set to sync in each account (by default, the inbox and drafts / sent / deleted), for every one of these, the app has to talk to the mail server when it checks mail
- The frequency of mail checks (by default, every 15 minutes)
- Push mail (by default, turned off at the account level) is more expensive than "check mail every ... minutes"
- How many messages you've set the app to sync (keep up to date with the mail server), there is an app level setting and optional per-account overrides:
App settings -> mail, receiving -> messages to sync
Long press on an account -> options and folders -> account options -> messages to sync
The larger the values, the more work the app has to do every time it checks mail or catches up to push changes. The default is 25 or 50 (for Android 5.0+).
- IMAP has compression, off by default, can be enabled per-account:
Long press on an account -> options and folders -> IMAP options -> compression
Should get about 50% compression / also faster on high latency mobile networks
- A per-account setting to preload attachments
Long press on an account -> options and folders -> preload
- Downloading attachments yourself
- Large messages (e.g. I've seen some promo type messages that exceeded a megabyte or more).
