A complaint is registered when someone marks the email they received as SPAM or Junk.
Complaint notifications are sent to the sender, the ISP provider, and Zift. When this happens, the recipient is given the status 'SPAM' in the contact list. Zift will add the recipient email address to the Suppression List for your account.
If your account is consistently generating a complaint rate higher than 0.1%, it is an indication that people are not happy receiving these emails and/or there is a serious problem with how you are creating your lists.
Accounts that generate a lot of complaints can cause many problems with delivery, especially if they are part of a shared IP pool used by multiple customers. These accounts may be blacklisted among Internet Service Providers as a known source of spam and/or suspended from sending future mailings in Zift123.