The Live team has now found the cause and provides tips to get the correct push-email behaviour of Hotmail. These issues only occur if you have your mail set to synchronize “As items arrive” in the “Sync schedule” settings and if you are also signed into Messenger in certain ways.
The workarounds for now:
– If you are signed in to Windows Live Messenger on your PC and your status is “Online” or “In a call” you could see interruption of push mail working on your device. The workaround is to have Messenger on your PC in any state other than “Online” or “In a call”.
So to those of you who have disabled “Show me as Away when I’m inactive for…” and/or “Show me as Busy and block my alerts when I’m running a full-screen program…” : turn it on again (Tools > Options > Personal)!
– If you are signed in to Windows Live Messenger on your Windows Mobile phone you could see interruption of push mail working on your device. The workaround is to sign out of Messenger on your Windows Mobile phone.