Did you try this:

*) Create a mail user used to store shared messages. Let's say it's share1@mydomain.com.
*) Send one email to share1@ first, to trigger Dovecot to create its mailbox on file system. Let's say its maildir path is: /var/vmail/vmail1/mydomain.com/share1/Maildir/ (it contains full INBOX, also other folders like Sent, Drafts, Junk, ...).
*) Follow our tutorial to create a public folder: http://www.iredmail.org/docs/public.folder.html

Instead of creating a new directory for public folder, you should create a symbol link to share1@'s mailbox as public folder. This way once share1@ gets new email (or other changes like deleting email, etc), all users who have privilege to access this public folder will get update.

Above step is mentioned in above tutorial:
http://www.iredmail.org/docs/public.fol … lic-folder

You missed a note in our tutorial:

I can achieve the sharing as mentioned in your first post with shared folder.

Here's a simple way to achieve it:

*) Create user "Ufficio1" (the one you mentioned in first post)
*) Login to SOGo as "Ufficio1".
*) Click the folder you want to share, there's a 3-dot menu at the right side of folder name. Click it, then choose 'Sharing'. Choose "Any authenticated User", then click the privileges you want to grant.

if you need to share multiple folders, just repeat above steps with different folders.

I need to warn you that SOGo has some problem with this sharing, check the issue i reported to SOGo team at July 26, 2016 below, help me to push SOGo team to fix it:
https://sogo.nu/bugs/view.php?id=3780

If you have Roundcube, you can enable its plugin "acl", then login to Roundcube, go to "Settings -> Folders", click the folder and set sharing privileges. It works fine.

You have to share all folders in the mailbox manually. If you add new folder, share it manually.

I have no good idea right now. sorry.
But what do you mean you cannot send email from "share1"? Isn't it one of your mail user?

Cannot login as "share1" to send email?

I have the exact same issue...

root@iredmail:/mnt/mailstore/public# mkdir .Test
root@iredmail:/mnt/mailstore/public# chown -R vmail:vmail .Test/
root@iredmail:/mnt/mailstore/public# chmod -R 0700 .Test/
root@iredmail:/mnt/mailstore/public# doveadm acl get -A "Public/Test"
Username ID Global Rights
root@iredmail:/mnt/mailstore/public# doveadm acl set -A "Public/Test" "user=postmaster@mydomain.com" lookup read write write-seen write-deleted insert delete expunge create
root@iredmail:/mnt/mailstore/public# doveadm acl get -A "Public/Test"
Username                              ID                                       Global Rights
postmaster@mydomain.com   user=postmaster@mydomain.com        create delete expunge insert lookup read write write-deleted write-seen
test@mydomain.com         user=postmaster@mydomain.com        create delete expunge insert lookup read write write-deleted write-seen
billy@mydomain.com        user=postmaster@mydomain.com        create delete expunge insert lookup read write write-deleted write-seen
rondaarchive@mydomain.com user=postmaster@mydomain.com        create delete expunge insert lookup read write write-deleted write-seen
adobe@mydomain.com        user=postmaster@mydomain.com        create delete expunge insert lookup read write write-deleted write-seen
theodosis@mydomain.com    user=postmaster@mydomain.com        create delete expunge insert lookup read write write-deleted write-seen
redfish@mydomain.com      user=postmaster@mydomain.com        create delete expunge insert lookup read write write-deleted write-seen
redchilly@mydomain.com    user=postmaster@mydomain.com        create delete expunge insert lookup read write write-deleted write-seen
fotis@mydomain.com        user=postmaster@mydomain.com        create delete expunge insert lookup read write write-deleted write-seen
redwine@mydomain.com      user=postmaster@mydomain.com        create delete expunge insert lookup read write write-deleted write-seen
marc@mydomain.com         user=postmaster@mydomain.com        create delete expunge insert lookup read write write-deleted write-seen
reception@mydomain.com    user=postmaster@mydomain.com        create delete expunge insert lookup read write write-deleted write-seen
redpepper@mydomain.com    user=postmaster@mydomain.com        create delete expunge insert lookup read write write-deleted write-seen
ronda@mydomain.com        user=postmaster@mydomain.com        create delete expunge insert lookup read write write-deleted write-seen
root@iredmail:/mnt/mailstore/public#

this is quite confusing... what is the username and what is the id?

also thunderbird and sogo do not show the folder and thunderbird subscribe waits forever without giving me any folders...

I'm trying to create three folders inside public. postmaster should have full access to all of them.
and then I want to add two more users with read permissions...

Any help?