Hi Chris,
When using the Method ListBuilder feature, what you are doing is considered a relay through your mail server, Method is relaying your message through the smtp protocol. We have found that some Service Providers by default block the ability to use this feature, also known as "Open Relay". This could be the issue you are experiencing, my suggestion would be to get more information from your Service Provider regarding this feature and make sure this is not the case for you, if so, get them to turn it off for your account, or do it yourself if you have access to it from your email account.
-Ben