construed to be spam itself, and the address is not added to the live mail list unless the recipient responds to the confirmation message. Egress spam filtering Email senders typically now do the same type of anti-spam checks on email coming from their users and customers as for inward email coming from the rest of the Internet. This protects their reputation, which could otherwise be harmed in the case of infection by spam-sending malware. Limit email backscatter If a receiving server initially fully accepts an email, and only later determines that the message is spam or to a non-existent recipient, it will generate a bounce message back to the supposed sender. However, if (as is often the case with spam), the sender information on the incoming email was forged to be that of an unrelated third party then this bounce message is backscatter spam. For this reason it is generally preferable for most rejection of incoming email to happen during the SMTP connection stage, with a 5xx error code, while the sending server is still connected. In this case then the sending server will report the problem to the real sender cleanly. Port 25 blocking Firewalls and routers can be programmed to not allow SMTP traffic (TCP port 25) from machines on the network that are not supposed to run Mail Transfer Agents or send email. This practice is somewhat controversial when ISPs block home users, especially if the ISPs do not allow the blocking to be turned off upon request. Email can still be sent from these computers to designated smart hosts via port 25 and to other smart hosts via the email submission port 587. Port 25 interception Network address translation can be used to intercept all port 25 (SMTP) traffic and direct it to a mail server that enforces rate limiting and egress spam filtering. This is commonly done in hotels, but it can cause email privacy problems, as well making it impossible to use STARTTLS and SMTP-AUTH if the port 587 submission port isn't used. Rate limiting Machines that suddenly start sending lots of email may well have become zombie computers. By limiting the rate that email can be sent around what is typical for the computer in question, legitimate email can still be sent, but large spam runs can be slowed down until manual investigation can be done. Spam report feedback loops By monitoring spam