This means that the IP from which the email is sent, that is, the one from your internet connection that your computer and all those connected to your office or home router use, is on a blacklist. Normally, this happens with dynamic IPs, it is possible that they have assigned an IP that has sent spam and has been banned.
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The way to solve this problem is by turning off your router for 30 minutes and turning it back on, this will change your IP and probably not be banned.
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Useful:
How to know my IP:
​http://ip.profesionalhosting.com
How to know if my IP is on a blacklist:
​http://www.profesionalhosting.com/soporte-en-linea/como-saber-si-mi-ip-esta-en-alguna-lista-negra-preg318.html