Thanks to the small SBankNotify additional app for the jailbroken iPhones you can finally get sound plus vibration notification when i.softbank.jp email is being received.
1) Add new Cydia source: http://homepage3.nifty.com/moyashi/cydia
2) Reload packages lists and find SBankNotify package, it will be installed togeather with the BossPrefs if you don't have it already.
3) Start the BossPrefs and turn the SBankNotify ON
If you want to change the incoming melody, swap the /System/Library/Audio/UISounds/sbank.caf AIFF file with your custom ringtone.