[exim] TCP Fast Open

For anyone interested in TFO, introduced in 4.90, I notice
that some 86% of outbound mails using a *.google.com host
used a TFO connection last week, from a MTA I have
access to.

A packet capture shows that they do send the banner before
getting the ACK for the SYN,ACK, too, and the connection
saves most of the maximum possible half-roundtrip time.
--
Cheers,
Jeremy