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If you are on shared hosting accounts, you will need to check and make sure that your ip address is not blacklisted on the major email filter systems. The problem with almost all shared hosting setups is they share ip’s among many sites. It could be a completely separate site using the ip that is blacklisted, which will affect you. Many hosts like dreamhost have entire blocks of ip’s that are blacklisted, and just because the domain name on the ip changes, it does not remove the blacklisted ip. I can almost guarantee that is the root cause of most people’s problems as I had the same problem years back. It was only after I went dedicated and purchased ‘clean’ blocks of ip’s that all my problems went away.