OK, line spacing can be customized! I was annoyed with the same problem, my google search led me here and someone mentioned the word "paragraph style" which gave me a good hit in the Outlook help.
See here the contents:
We follow these instructions in a new e-mail message and then click "default" and say OK, and hopefully you should never be bothered by double spaced e-mail messages again!
Outlook > E-mail > Customizing messages
Change the line spacing in e-mail messages
When you compose an e-mail message in Microsoft Office Outlook, you can change the spacing between the lines and the spacing before or after each paragraph. However, line spacing cannot be configured for plain text messages.
Change the line spacing
Line spacing is the amount of space between the lines within a paragraph. The default line spacing setting is single spacing.
Select the text that you want to change.
On the Format Text tab, in the Paragraph group, click .
On the Indents and Spacing tab, under Spacing, under Line Spacing, select a spacing option from the list.
Change the spacing before or after paragraphs
Select the paragraphs in which you want to change the spacing.
On the Format Text tab, in the Paragraph group, click .
On the Indents and Spacing tab, under Spacing, do any of the following:
In the Before box, type or select the spacing that you want above the paragraphs.
In the After box, type or select the spacing that you want below the paragraphs.
Note Paragraph spacing is measured in pixels.