Brian Tillman said:
Use Shift-Enter for your lione breaks, not plain Enter.
To provide some further info, hitting Enter adds a <P> paragraph tag.
The extra space you see is not an extra line but increased spacing
between paragraph. Hitting Shift+Enter adds a <BR> line-break tag. As
an example, I write in Word (in HTML/web format):
This is a paragrah.<Enter>
This is the next paragraph.<Shift+Enter>
Followed by a line.<Enter>
What I get after e-mailing this test message is (shown simplified and
with indentation added to provide separation of tags and content):
....
<p>
This is a paragraph.
</p>
<p>
This is the next paragraph.
<br>
Followed by a line.
</p>
The <P>...</P> tag pair denotes a paragraph, and paragraphs get a bit
more whitespace between them. <BR> is a line break. Even blank lines
have whitespace between them but just not as much as paragraphs.