How to fix mail() double line spacing in message body?

So, I don't quite get how to solve this. See code example below,
which shows "normal" CR/LF. The
basic problem is that emails that my form-to-mails generate using the
mail() function consistently put in two CR/LF's for every apparent CF/
LF. How does this happen, how to fix the...

Double line spaces

that appear

like this in the body?

-Chuck

$message ='Dear ' . Trim($fullname) .',
Thank you for your interest in our product.

Blah blah blah blah blah

Sincerely,

Joe Schmoe';

$headers .= 'From: Web server <w...@domainname.com>' . "\r\n";
$subject = 'Your posted message';

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The example above would generate a message that reads like:

Dear Mr. Smith,

Thank you for your interest in our product.


Blah blah blah.


Sincerely,


Joe Schmoe.