To make Outlook more tolerant of sending code snippets and paths around between programmers is there a way to tell it that things like . and / and _ are valid word separators?
If you, like me, are using Microsoft Word 2007 as your Outlook email editor, then you are limited to what Word's spell check supports. Although it may be possible to hack away at the underlying code, I don't think this is possible as you are hoping. I have had good luck with The Daily Reviewer for Office questions and the discussion of Spell Check - Treat Underscore As Word Separator suggests that the best you can hope to accomplish is have the spell check ignore words containing underscores. The discussion of adding all variables to a custom dictionary seems foolish to me, although perhaps someone could automate a routine that adds them by looking up individual words.
I'd probably dig into the other option mentioned: "setting the language property of these words to 'no proofing' (you could use a character style for this)." The article at Exempting specific text from spell checking tells how to create the character style, but I'm not sure you can automatically apply the style, once created...