AuthenticationFailedException using SecureServer

We've got ScriptMaster installed in Filemaker Pro 11 on a client's PC. When I attempt to send an HTML-formatted email using SendEmailWithAuthentication, I get a failure message that reports the details as javax.mail.AuthenticationFailedException. The SMTP server we're using is SecureServer.net, so there are clearly differences from the telco mail server I tested the script with off-site, and which sends the email fine.

Since SecureServer is clearly different from my testing environment, do I need to do something special to send mail through it? What assumptions are made by the ScriptMaster function that might be violated by how SecureServer is set up? Do we need to use a different solution?