A script creates e-mails in specific folders (on a self hosted mail server) in 15 Minute intervals. The iOS mail client should download emails inside those folders while connected to Wi-Fi so all the emails are up to date when there is no Internet connection available anymore.
The mail app is able to fetch new emails while the app is opened, but it's not able to sync while the app is not active.
I also the tried using other mail apps like Outlook but the results are still the same.
I have tried setting the fetch schedule under Settings → Passwords & Accounts → Fetch New Data to every 15 minutes. Sadly it still does not sync the folders.
Do you guys know if it's even possible to do something like this on iOS devices?
Edit: Sometimes it seems like the mail client syncs the e-mails at 5:15 am. Sadly it only syncs the e-mail subject and not the e-mail content. I would need a internet connection to download the content. Why are the e-mails only synced at 5:15 am and not every 15 minutes?
I tested this on an iPhone 6s and iPhone X with iOS 12.3.1