From Forum Nokia Wiki

There is an API for controlling Email LED on S60 devices( available in the new E-Series plugin SDK ). The main Purpose of this API is to control Email LED of device via Publish&Subscribe API. So the 3rd party applications can set Email LED blinking but cannot stop blinking programmatically. Any user activity stops the blinking LED indicator.

Here is the code snippet for demonstrating the functionality:

Header file: EmailLedValues.h, e32property.h
Library: euser.lib

TInt retVal = KErrNone;
 retVal = RProperty::Set( KPropertyUidEmailLedCategory, KEmailLEDOnRequest, KSetEmailLedOn );
 if( retVal != KErrNone ) 
 {
 switch( retVal )
    {
      case KErrNotFound:
        {
         // Property was not defined. Device does not have an Email LED.
        CEikonEnv::InfoWinL(_L("No Email LED indicator"),_L(""));
        }
        break;
       default:
        {
         // Some other error.
        }
        break;
      }
    }

The URL for downloading E-Series plugin SDK for 3rd edition and 3rd edition FP1 devices is given below:

http://www.forum.nokia.com/info/sw.nokia.com/id/c136eec5-5496-43cc-be53-83a7a6ed8a31/Nokia_Eseries_SDK_Plug-Ins_for_S60.html