blackhole all mail except for one domain using postfix

postfix_blackhole.sh

# define a transport for mail you want to deliver
echo "$VALID_DOMAIN smtp:[$VALID_RELAYHOST]" > /etc/postfix/transport
postconf -e transport_maps = hash:/etc/postfix/transport

# everything else goes to a bogus relayhost, which causes a bounce
postconf -e relayhost = 169.254.0.0

# immediately throw away all bounces
postconf -e maximal_queue_lifetime = 0
postconf -e bounce_queue_lifetime = 0

# optionally record all blackholed mail to a specific address
# postconf -e always_bcc = blackhole@$VALID_DOMAIN

# time for Jell-O(tm)
postmap /etc/postfix/transport
postfix reload