Our's is a paid for service, so the only emails we create have been asked for (e.g. User Log In / Password reset emails).
I see this message has been posted in the past, but the link for Bulk Sender no longer works:
Check your IP is blacklisted I had the same issue and found my sons PC was sending out masses of spam without any of us realising. Spamcop is a good service to check the issue.