I have recently set up outgoing email on a clients 5.0.6 box with
sendmail (sco's sendamil)
Originally, it sent mail directly, but we soon discovered that several
large important domains reject mail from certain ip addresses
(aol/earthlink/etc reject smtp from dialup/dsl/cable addresses)
So I changed sendmail to use the isp's (comcast) smart-host.
Fine now mail goes out and is received everywhere.
Now however, undeliverable mails are not generating notifications to
root, or any other form of notification I can find.
The customer very strongly wants to be notified when an attempted
outgoing email fails for whatever reason. I educated them ...