Question: how to check if you are a blacklisted sender?
Answer: multi.valli.org/lookup. This is one of our favorites because it's pretty comprehensive and checks 120+ blacklists.
www.mxtoolbox.com/blacklists.aspx. MXToolbox is free. ... 
www.dnsstuff.com/tools. You can check your IP address and your domain here for free. ... 
www.dnsbl.info.