The emails that are sent from the PowerStore make use of the standard PHP Mail function and this does not support authentication.
The only form of email authentication that we offer is though PEAR Mail, but this method is only available with the Universal Email tool and is not included in the Power Store. Does your host offer access to a local SMTP server that does not require authentication? This would be the best way forward.
It is not necessarily a straight forward solution but it should be possible to create this functionality if you own the Universal Email tool. The idea would be that you recreate each of the Universal Email server behaviors in the site and make use of the PEAR Mail so that you can use authentication. If you are interested in this as a solution here are links to the Universal Email pages on our site so that you can take a look: