A "Complaint" is also known as an "Abuse Report" is when a recipient marks an email as "Spam" or "Junk", effectively moving the email from their inbox to their Spam/Junk folder. Or when a recipient Unsubscribes and in the process confirms that they "did not ask to receive this email".
Both events are tracked as a Complaint and will register on your account as such.
Acceptable Industry Standards:
The email industry has firmly set 0.1% as the maximum acceptable rate of complaints. This is calculated by dividing the number of complaints by the number of emails that were sent. ((complaints/sent) * 100)
However, many ISPs can start to stop sending your email after just 0.01%.
That's why we place so much emphasis on only loading people who will welcome your contact and not complain.
The best practices are to add only people you just met at a meeting, etc, and limiting loading of your past contacts to those who you have corresponded with within the last 6 months is so important. When you meet people, do yourself and us a favor, let them know you'll be sending them an opt-in for your newsletter so be on the lookout for it.