Yahoo, EarthLink to Test New Anti-Spam System  WASHINGTON (Reuters) - EarthLink Inc. &lt;A HREF="http://www.reuters.co.uk/financeQuoteLookup.jhtml?ticker=ELNK.O qtype=sym infotype=info qcat=news"&gt;ELNK.O&lt;/A&gt; and Yahoo  Inc. &lt;A HREF="http://www.reuters.co.uk/financeQuoteLookup.jhtml?ticker=YHOO.O qtype=sym infotype=info qcat=news"&gt;YHOO.O&lt;/A&gt; said on Monday they would begin tests of a new  anti-spam technology that encodes digital signatures into  customers' e-mail as a way to separate legitimate messages from  unwanted spam.