How do I debug run time errors

I occasionally get a run time error on my code (picture of the error message attached) when this comes up the only option I have is 'exit' which doesn't allow me to see where in the code the error happened. This is strange as it has only recently started happening on code I have been using for a long while and it doesn't always happen, I can run the code numerous times without a problem then every now and again it will occur. I have done some searching and I think it has something to do with when the code accesses outlook to write an email and judging by the place the code stops this guess fits. I have tried stepping through my code but then no error occurs.

Is there a specific way to debug or track down this error?