Are you running exim?
I have the following in my config, which seems to help the issue:
# disable TCP fast open for gsmtp servers, due to a kernel bug; the bug was
supposedly
# fixed in 5.18, but it seems there's still some quirkiness with gmail
without this
# setting
#
#
https://www.chromosphere.co.uk/2022/06/01/googles-tcp-fast-open-breaks-exim-delivery/
#
https://forum.hestiacp.com/t/gmail-smtp-timeout-after-sending-data-block-connection-timed-out/4745/75
#
https://github.com/myvesta/vesta/commit/c7aa6ec7494bc3b7ac4885182383c1bf88692c1b
hosts_try_fastopen = !*.l.google.com : !*.googlemail.com : *
Good luck,
--
Alex
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