BLUF
According to Wikipedia:
BLUF (bottom line up front) […] is a paragraph where the conclusions and recommendations are placed at the beginning of the text, rather than the end, in order to facilitate rapid decision making.
Which seems to me to be pretty much the same approach that a lot of people take by writing “tl;dr: [summary of email]” at the top of an email.
I think it’s a good approach, but I tend to prefer to write Summary: or Short version: instead of using an abbreviation.