David,
I think all hosts and even 100% opt-in lists experience blocking from time to time. Yes, you would know more about the exact type(s) of blocks you are facing with the ListMail bounce feature and "notify when a user is removed for bouncing" enabled for your list(s) - a large portion of the bounce message is returned in the notification which should also contain the reason the message bounced.
Apart from reading the bounces your host can scan the system mail logs for URLs (returned by most blocked servers) to identify blocks against your server. They can then visit those URLs and use any removal procedures available.
PS. I got an email form a company promising to send millions of emails. In your experience, what would you say about http://www.emailadvertisinginc.com/
In my experience email list-providers are a bad idea. Most companies, at the very least, sell their lists to more than 5 email marketers. Others might go so-far as to harvest email addresses from the internet which can often include "honeypot" addresses that are very bad for your servers reputation just as are all the additional complaints.
Please let me know if I can be of further assistance!
Regards
