The IP address of the inbound connection has been denied access in the SMTP Access Control properties of the MailEnable Administration Program or has been automatically banned because of too many subsequent invalid commands. The incoming IP address can automatically be added to the inbound "Access control" list if the SMTP security "connection dropping" option is enabled. The SMTP service will drop the connection when the failed number of commands or recipients is exceeded and add the IP address to the inbound "Access control" list.