As you probably already know, Microsoft removed Internet Explorer (IE) as the tool for rendering HTML messages as of Outlook 2007. In addition, the Microsoft Office Outlook editor was replaced by the Microsoft Office Word 2007 editor for composing email messages. This has caused some minor inconveniences for people who compose HTML-formatted messages.
Learn more: Outlook 2007: HTML Forms Are Out; CSS Is In
I’ve talked with many users whose messages appear to have extra empty lines between text where just a single blank line was intended. This occurs in Microsoft Office Outlook 2007 when a user composing an HTML email message hits the Enter key after completing a paragraph. Users might also hit Enter to advance the cursor with the intention of leaving a blank line between paragraphs. The problem arises with Word 2007 generating <p> paragraph tags in response to the Enter request. In HTML, the <p> tag is rendered as double-spaced. This was not an issue in Outlook 2003, even when Word 2003 was used as the email editor. The issue is especially consistent when the recipient is using a web-based mail reader such as Gmail or GMX.com.
The simplest solution, if this is perceived as an issue at all, is to use Shift+Enter when ending a paragraph or creating space between paragraphs. The Word 2007 editor built into Outlook 2007 will generate a <br> tag for Shift+Enter. The <br> tag represents a line break in HTML and won’t be rendered as a double-space when viewing the HTML-formatted message.
Learn more: Outlook 2007's Fancy Formatting: A Quick How-To