Spam with single URL in the body

[Le Vu]

Hi all,

Recently we are received a lot of spam with a single link in the body.
These spams come from valid Yahoo accounts. Those links are not listed
in any URI blacklist.

My question is how to write a rule/plugin to detect these kind of spams?

Your suggestion is much appreciated

Thank you

Here is what the spam looks like:
....
X-Mailer: YahooMailWebService/0.8.107.285259
Date: Sat, 18 Dec 2010 15:56:29 -0800
...
X-MailScanner-SpamCheck: spam, SpamAssassin (not cached, score=10.202,
	required 7.5, BAYES_99 6.50, DKIM_SIGNED 0.10, DKIM_VALID -0.10,
	DKIM_VALID_AU -0.10, FREEMAIL_FROM 2.00, FSL_RU_URL 0.00,
	MISSING_SUBJECT 1.80, RCVD_IN_DNSWL_NONE -0.00, RFC_ABUSE_POST 0.00,
	TVD_SPACE_RATIO 0.00, T_RP_MATCHES_RCVD -0.01,
	T_TO_NO_BRKTS_FREEMAIL 0.01)
...
Return-Path: bwestuser at yahoo.com
MIME-Version: 1.0

http://obermaneken.ru/christmas.php?yahooID=457