The "Local Domains" are the domain names that your mail
system considers local. Messages to "Local Domains" are also called "Incoming".
Messages to domains not in "Local Domains" are also called "Outgoing".
Messages from "Local Domains" are called "local" provided they come from IP's in "Accept All
Mail" or are authenticated. ASSP supports also "POP before SMTP".
Messages from domains not on in "Local Domains" are called "external".
That a mail claims to be from one of your local domains does not
allow it to be relayed -- this is easily spoofed and not useful as a
security measure. Spoofing an IP address is more complicated in this
type of environment, and generally relaying is limited by IP
If your clients dialup or are dynamically assigned from an untrusted
pool, then the only reliable way to allow relaying is through
AUTHENTICATED smtp, and your mail handler must support this type of
authentication, and you must enable it in your clients. ASSP
recognizes authenticated connections and allows them to relay.
Not all ISPs will allow their customers to connect to your SMTP
port. Many block connections to port 25 (except to their own mail
server) to prevent spam. ASSP therefore provides a "Second SMTP Listen Port".