Have you seen Mail::Bulkmail? The module itself doesn't always work, but you can at least learn about bundling mail in envelopes :) The trick is that if you have 50 foo.com addresses and the email itself is identical, you don't need 50 connections.

But if 500 messages is causing a problem, consider looking for another ISP or skip the ISP and send things directly over SMTP (install a local mail server and use it.)