My email body has everything the "optimal" reader (i.e., someone who is already familiar with the topic item) needs to |action_verb| regarding what I've written.

Then I "conclude" with my usual sign-off (e.g., Cheers, [CRLF] "/Aceyman").

BELOW that—but above my default signature/contact info block—I place all the deeper-dive / expository / rationalization narrative which provides that level of detail for anyone who needs more than my main body provides.

I've been using this pattern for about a year or so with good results.

/Acey

At the start of the supplementary section—and depending on the topic & audience—I'll sometimes use the 'section title' Extra-credit reading, because everyone loves a chance for some extra-credit, right? That's my little psych ploy to entice readers to take a look at the nominally 'optional' expository stuff.