zlib plugin, as far as I know, only supports zstd, gzip, bzip2 and lzma/xz compression. The last one is being deprecated.

I have found this interesting post in the mailing list:

https://dovecot.org/pipermail/dovecot/2021-February/121329.html

Same problem here with Dovecot 2.3.13, "doveadm altmove -r" is not
moving anything from alternate to default storage. I fixed this by
reverting this commit:

https://github.com/dovecot/core/commit/2795f6183049a8a4cc489869b3e866dc20a8a732

Is this fixed in 2.3.14 ? Does doveadm altmove -r works as expected in 2.3.14?

Regards

Sent: Sunday, March 21, 2021 at 11:28 PM
From: "justina colmena ~biz" <justina@colmena.biz>
To: dovecot@dovecot.org
Subject: Re: Question about doveadm altmove

On Sunday, March 21, 2021 12:16:28 PM AKDT María Arrea wrote:
> Hello.
>
> We are running dovecot 2.3.13. Full doveconf -n output below
>
> In 2.3.14 Changelog I found this:
>
> * Remove XZ/LZMA write support. Read support will be removed in future
> release.
> We are using mdbox + XZ/LZMA for alternate storage (messages older than 2
> weeks are moved to ALT storage via cron job), so we must convert from XZ to
> another thing (maybe zstd or bz2).

Why can't you just pipe the output of "doveadm altmove" command through an
external command to do the XZ/LZMA compression if dovecot no longer supports
it internally?

>From doveadm-altmove (1):
> This command can be used with sdbox or mdbox storage to move mails to
alternative
> storage path when :ALT=<path> is specified for the mail location.

And that's set in stone.

https://en.wikipedia.org/wiki/XZ_Utils

So what are the issues with xz? Security? Crashes or viruses on expanding
invalid archives?