Exim 4.93 changed the default for the "hosts_try_fastopen" transport
option to be "*", and the default for the
net.ipv4.tcp_fastopen_blackhole_timeout_sec sysctl changed from 3600
(i.e. an hour) to 0 at some point between the kernel versions in Debian
buster (10) and bullseye (11).
A workaround is to add something similar to "hosts_try_fastopen = !
*.l.google.com" to your SMTP transports.
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