Hi John,
550 Administrative prohibition
Does this mean that my new server is restricting this type of activity?
Yes, this is likely. I recommend asking your host if they have any hourly or daily limits on email. I also recommend letting them know a little bit about how you acquire leads, etc. so they feel safe allowing you to send email - maybe they can raise your limits.
Another possibility is you are not using the correct SMTP hostname or authenticating properly with the server. If you're not using it already, try enabling the AUTH setting with a username and password.
Yet another possibility, although it's unlikely, is that your server wants the "Send From Email" to be the same as the "Bounce to Email". Even less likely is that the account used for AUTH must be used as the "Bounce to Email" and/or "Send From Email".
Please let me know if I can be of further assistance!
Regards