Hi Yves
I'd say there are a couple of reasons for my (personal) preference of the present perfect in the examples you gave earlier:
1. The past activities I'm mentioning in the email probably took place extremely recently, and the fact that I'm currently writing about those very recent events provides an
extremely direct and clear connection to the present and me.
2. Precisely
when I spoke to my boss or precisely
when I forwarded you mail is not important. It is only important
THAT those things were done.
As I said in my last post, however, I can also imagine an American using the simple past tense.
To give you a different sort of example, imagine that someone shot the US president. I cannot imagine any American running around and giving people the news by saying "The president has been shot!" Instead, I would expect the simple past tense: "Hey! Did you hear? Some yahoo shot the president!"