Jason sent in a comment on a prior post discussing deliverability asking about web auto responders. This is a distinction that is important if you are not aware of your IT infrastructure – the mail that gets generated when someone fills out a form on your site may be coming from a different place than your bulk email.
This means that you may get differing deliverability rates, and may have some other strange issues. Although I have not seen any email providers offering this service, I have seen it bundled in CRM systems such as SalesForce.com
I know… Deliverability is not the sexiest stuff…