This can happen for a number of reasons, often the cause is actually a virus on a computer attached to your network which would make it send out spam emails. After you have tracked this down though you will want to get the IP removed from the blacklists. To do this you should first look up your IP here: https://www.spamhaus.org/lookup.lasso.
If it tells you that you IP is blacklisted it will also give you a reason, and the appropriate steps for you to take to get this removed from the blacklist.
