Yes... I get that 'internal email' too! But I usually just leave the idea to ruminate in my head for a couple of weeks. Many die away, but the good ones remain and become better.
It's a method I learnt from Stephen King, in the introduction to his book 'Nightmares and Dreamscapes'.
I write down as many of my ideas as I possibly can. I can't count the times I've thought of an idea for a great story and it disappeared before I could write it down. Then again, I have notebooks filled with ideas I wrote down and haven't used yet :)