-# If all works you'll not see any difference. To enable in the
-# kernel, 'sudo sh -c "echo 3 > /proc/sys/net/ipv4/tcp_fastopen"'.
-# A packet capture on the loopback interface will show the TFU
-# option on the SYN, but the fast-output SMTP banner will not
-# be seen unless you also deliberately emulate a long path:
-# 'sudo tc qdisc add dev lo root netem delay 100ms'
-#
-# If the client-side is disabled in the kernel, Exim logs
-# will become noisy.
-#
-exim -DSERVER=server -bd -oX PORT_D