In an effort to reduce
email spam originating from their customer's
IP addresses, some
internet service providers (ISPs), will not allow their customers to communicate directly with recipient mailservers via the default SMTP
port number 25. Instead, often they will set up a smarthost to which their customers can direct all their outward mail – or customers could alternatively use one of the commercial smarthost services. Sometimes, even if an outward port 25 is not blocked, an individual or organisation's normal external IP address has a difficulty in getting
SMTP mail accepted. This could be because that IP was assigned in the past to someone who sent spam from it, or appears to be a dynamic address such as typically used for home connection. Whatever the reason for the "poor reputation" or "blacklisting", they can choose to redirect all their email out to an external smarthost for delivery. ==Reducing complexity==