You can search various blacklists to see if your email server is flagged [the link points to a site that checks 100+ blacklists]. Once you figure out what service (if any) has you flagged, check their website to see how to get unblocked.
You generally have to search using the mail servers' IP addresses. The easiest way to find these is to just ask your ISP (since they host your mail).
Alternately, you can run a mail server lookup for your domain name (everything after the @ in your email address). The mail servers will show up marked as "MX" and have a number (usually a multiple of 10) in front of them. If the mail (MX) record is another name (not an xxx.xxx.xxx.xxx number), then you'll have to look a little further down in the record to see what number matches with that name. (It's really not as hard as it sounds ;-))