Many providers will use a shared IP address for their services. If other users are sending mail out from the same IP address as you, their sending patterns could be the reason for the poor reputation.
You could also have malware on your PC or network which is sending spam.
If you’re sending newsletters then you may have issues with your mailing practices that are causing recipients to report your mails as spam. You should review your sending behaviour and apply proper list hygiene to improve this.
If you’re a service provider, it is possible that you have taken on a spammer as customer.
You could have a compromised user account being used to send mail. Checking your mail logs for unusual activity should allow you to determine this.