These days, service providers like British Telecom have tightened up their requirements on email delivery. Most ISPs now accept email only from servers that can authenticate through SMTP. Sendmail has authentication built in, allowing administrators to use an “authinfo” file, for example. The server then authenticates every time it sends mail out to the ISP.
That’s great for reducing spam. But it can have a side-effect of killing local mail on the server. Here is an recent example from Debian Squeeze. The authentication features that allowed this server to successfully send mail through the ISP were preventing local mail from working: Continue reading