If you want to deal with someone, say the message in the message.
If you don’t (“see attached”) then I suspect 1/3 worse conversion rate on loose connections you are trying to convert to an action.
For different reasons, long emails suck. Easy to read, understand, fast, and react to/reply/act on is best.
If “I have to do it later” then it will be a while. If I can make the move on iPhone on the stairs up the subway, I will.
Write emails that go in GTDs “do now” category.