- Synchronizing all folders can cause a lot of data to be transmitted.
- How exactly do you accomplish this (What phone model are you using? What’s the functionality called)? Do you disable “mobile data”? In that case K-9 Mail will happily use a WiFi connection if available.
K-9 Mail should not check mail if there’s no network connection. However, it will use any network that is available.
The app will usually also respect the sync interval. However, K-9 Mail will retry after 5 minutes and then in increasing intervals if the previous mail check has failed.
You can gather more information by record a debug log. See LoggingErrors · k9mail/k-9 Wiki · GitHub
PS: There is an email app called KMail. Depending on the context it can be very confusing if you refer to K-9 Mail as “kmail”.