If you wonder sometimes why your subscribers or partners don't receive email messages from you - check your Ip or domain. It can be blacklisted by various mail servers, ISP's, etc.
Here is one main source to do this: http://www.blacklistalert.org. Just go there and type in your IP or domain name. It will show you the list of various service providers and status of your IP in their blacklists. A must have tool for those, who send's out many messages.