Same issue.

*** YMail SOAP Response ***
<?xml version="1.0" encoding="UTF-8"?>
<SOAP-ENV:Envelope xmlns:SOAP-ENV="http://schemas.xmlsoap.org/soap/envelope/">
<SOAP-ENV:Header/>
<SOAP-ENV:Body>
<SOAP-ENV:Fault>
<faultcode>SOAP-ENV:Client.InvalidFromAddress</faultcode>
<faultstring>You are not allowed to send as "user"@yahoo.com</faultstring>

<faultactor/>
</SOAP-ENV:Fault>
</SOAP-ENV:Body>
</SOAP-ENV:Envelope>

I have an "user"@yahoo.com.ar not "user"@yahoo.com, could be this? I have another ymail.com and it works ok.