Thunderbird for Android / K-9 Mail notifying you late (or never) when an e-mail is sent to you?

What's going on?

By default, neither application enables IMAP IDLE (also known as IMAP push).  Under most circumstances, this means your e-mail application will only notify you when it manually wakes up the phone to check for new mail.  That can be up to 30 minutes in the default configuration.

Although I would have imagined enabling IMAP IDLE ought to be default behavior, sadly it isn't.

How to fix it?

It's surprisingly easy:

• Launch the app.
• Tap on the hamburger menu (top left) to then tap Manage folders.\
• Tap on the folder you want to get updated as soon as possible.
• Select the checkbox Enable sync.
• Select the checkbox Enable Push.
• If you want to get notified, enable the checkbox Notifications.

That is it.