I all of a sudden had my email returned and was told to check IP address. My IP was not blocked and still cannot send to this address, works fine to everyone else - What could be the problem?
Without going too deep into this, perhaps the one email address you're trying to send to blocked you. If that's known to not be the case, there could be a number of issues starting with who hosts your email servers. Are those IP addresses clear? You can check here: https://www.whatismyip.com/blacklist-check/