Carriage returns get turned into two

When I send out a mail with two carriage returns between paragraphs (the standard), they are received by my e-mail client as 4.

The result is silly looking e-mails.




Like this.




With big space between paragraphs.

Looking at the source of the e-mail, the line breaks are hex code 0D and 0A (\r followed by \n).

I'm using a Linux box to host my Drupal site. My Thunderbird mail client is on an XP box. I've tried to track down where in the code a line break gets converted into two but without success. Any help is appreciated.