An em-dash can generally be inserted in most word processors by hitting a space, typing a hyphen, hitting a space then typing the next word then hitting space.  The hyphen (or en-dash) gets converted to an em-dash when the space is hit after the word following the hyphen you originally typed is pressed.  It has worked this way in MS-Word for as long as I can remember now, though the last version that I used for years before 2016 was 2010, and I have no specific memory of what Word 2007 did.  If you use double hyphens (with no spaces) these get converted to an em-dash whether you have spaces before the double hyphen or after the double hyphen but the conversion still happens at the same point in time, when you hit the space bar after the word that follows the double hyphens.

It's impossible to say whether this works in e-mail as most clients will allow you to tweak all sorts of settings.  Outlook is set up by default to do the double dash conversion to an em-dash.

For anyone curious about the difference between an en-dash and em-dash, and when each should be used, see any one of the search results from:  https://duckduckgo.com/?q=what+is+the+difference+between+an+em-dash+and+an+en-dash
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Brian - Windows 10 Pro, 64-Bit, Version 1809, Build 17763  

Growth for the sake of growth is the ideology of the cancer cell.

           ~ Edward Abbey