Emails from a sender are blocked by the Anti-Spam service stating that they are in your company's blacklist.
553-Sorry, your email address has been blacklisted
553-Sorry, your domain has been blacklisted
553-Sorry, your IP address has been blacklisted
The sender is listed by email address, domain or IP address on the Global, Group or User Blocked Sender's List. Group and User levels are not enabled by default, so this feature must have previously been enabled for this to be a possibility.