It's because it doesn't move email by email to alt storage, but the
whole file. So the emails move as is, be them compressed or
uncompressed.
I think you could do what you want, but passing the correct options,
but I don't think it would be easy to do.
I would just store them compressed all the time, saves disk space, io
bandwidth, and if it goes over the network, actually will give a huge
performance improvement.
I have moved all mine to zstd currently, from gzip I had been using
since it was possible.
Quoting Alex King <alex at king.net.nz>:
> Using 2.3.13 (89f716dc2) (debian package), with mdbox storage.
>
> I considering adding the zlib plugin.? I understand that reading
> mail can cope with either compressed or uncompressed mailboxes.
>
> I'm wondering if I can leave the main storage uncompressed, while
> having the alternate storage compressed.? There does not seem to be
> a way to configure this, from
> https://doc.dovecot.org/settings/plugin/zlib-plugin/ it seems
> compression (on saving emails) is an all-or-nothing option?
>
> Can I leave zlib_save unset in my main configuration, and have
> doveadm use some alternate configuration with zlib_save set?? It
> doesn't look hopeful because I don't see the ability to use a
> different config at https://wiki.dovecot.org/Tools/Doveadm/Altmove.
>
> Perhaps I would need multiple copies of the daemon running?
>
> Or is it better just to compress everything?? I plan to store older
> mails (perhaps 3months+) in the alternative storage, and was
> thinking it would be nice to have those compressed, while I don't
> need to compress the main store.
>
> Any feedback welcome.
>
> Thanks,
> Alex