One of the monthly invoices I get by email (from those nice people at VoIPtalk) starts with:
Please view this with a fixed-width font.
And indeed, all the columns line up much more tidily if you do that, but come on, chaps, times have moved on! Even people like me are no longer using Pine to read their email, and you can bet that the vast majority of your customers won’t even know how to set their email reader into a fixed-width font.
Ironically, if you want people to view your message in Courier nowadays, you probably need to send it as HTML!