As correctly said the envelope is where possibility of successful start is demonstrated. So if care is taken to keep parameters (egt) within limits it can be attempted even outside the envelope. But let us see why would a pilot attempt that. Engine doesn't fail without a reason and when FADEC detects a sub idle condition it attempts an auto relight for 30 seconds varying fuel flow etc. If the engine was good enough to start it should have relit. Since it hasn't there is no point wasting time to relight outside the envelope because there are other things to do, some decisions need to be taken. By the time you do that you would be closer to the envelope.