In /usr/share/doc/exim4-base/spec.txt.gz :
--------
In order to append to an NFS file safely from more than one host, it is
necessary to take out a lock before opening the file, and the lock file
achieves this. Otherwise, even with fcntl() locking, there is a risk of file
corruption.
--------

I don't see why. If used correctly, fcntl() locking should be
sufficient under NFS. And if fcntl() locking is broken, lock files
won't prevent corruption anyway: even if you have the lock, there
is no guarantee that the client will synchronize the changes with
the server before another process gets the lock and do changes
(this is the problem I had with shared zsh history, which often
got corrupted even though zsh used some form of locking... until
I patched it in 2008 to use fcntl() locking).

[...]

Hello Vincent,

Would you mind taking question upstream (exim-users mailing list)?

This does not seem to be a bug report but question that would best be
discussed directly.

cu Andreas

Hi Andreas,

Done 9 hours ago, but it seems that the mailing-list has some issues.

Well, the spec seems to be incorrect. So that's more than a question.
But let's first see what will be said in the exim-users mailing-list.

IMO, Debian is not in a position to change the upstream docs. That's a
thing best done upstream.

Greetings
Marc

Yes, but I don't know what to do. Even though my mail to exim-users
was accepted [status=sent (250 OK id=1jYqvc-0001jm-6G)], I can't see
it on <https://lists.exim.org/lurker/list/exim-users.en.html>.

On 2020-05-14 Vincent Lefevre <vincent@vinc17.net> wrote:
[...]

No worries, iirc exim-users is moderated for non-members. It should go
through.

cu Andreas

Indeed, it had to be approved (note that I subscribed before I posted,
but perhaps this is because this was my first post to the list).

The thread should be available there:

https://lists.exim.org/lurker/message/20200513.125602.7be7983c.en.html