mail-sink is a utility program that implements a "black hole" function. It listens on the named host (or address) and port. It accepts Simple Mail Transfer Protocol (SMTP) messages from the network and discards them.
Written in Go: http://www.golang.org/
Install with:
go get github.com/stone/mail-sink
Usage:
$ ./mail-sink -h
Usage of mail-sink:
-H="localhost": hostname to greet with
-i="localhost": listen on interface
-p=25: listen port
-v=false: log the mail body