[exim] seemingly always-zero part in queue-ids

Hello,

I have observed that on multiple machines where I've seen exim running,
the queue-IDs usually contain three zeroes:

1S5ff6-000FQ0-Kb
1S5fkq-000N3Y-8x
1S5fko-0002KT-P2
       ^^^

What's the purpose of these zeroes? I.e. why are they even here? Is this
an architectural/OS specific thing? (I expect most machines running exim
to be Linuxes, but FreeBSD also has three zeroes here..)


Best regards,

    Moritz