I just tried this solution and for me it work great.

Here are the changes I made :

 
Main.cf

Replace : virtual_alias_maps = mysql:/etc/postfix/mysql_virtual_alias_maps.cf
For this : virtual_alias_maps = mysql:/etc/postfix/mysql_virtual_alias_maps.cf, mysql:/etc/postfix/mysql_virtual_alias_maps_custom.cf

Copy /etc/postfix/mysql_virtual_alias_maps.cf to mysql_virtual_alias_maps_custom.cf.

mysql_virtual_alias_maps_custom.cf

Replace : query       = SELECT goto FROM alias WHERE address='%s' AND active='1' AND expired >= NOW()
For this : query       = SELECT username FROM mailbox WHERE memberOfGroup ='%s' AND active='1' AND expired >= NOW()

And in the mySql database just add the field 'memberOfGroup' on the mailbox table.

Ok, but using that method I have to add all the users, one by one, or can I do some kind of regex like *@domain.com ?

Not tested yet. Why not give it a try by yourself?

PS: i think you can add 'all@domain.ltd' during adding 'memberOfGroup' column:

mysql> ALTER TABLE mailbox ADD memberOfGroup VARCHAR(255) NOT NULL DEFAULT 'all@domain.ltd';

Note: Maybe VARCHAR(255) is not what you need, change it to the type you need.

# table: vmail.mailbox
# Columns: username, memberOfGroup
www@domain.com all@domain.com

And add one more virtual_alias_maps lookup file, make it search memberOfGroup column.

PS: you have to add 'memberOfGroup' column manually.

Is it possible to create an Alias like "all@domain.com" to send an email to all the users of a domain ?

I tried creating an alias on Postfix.Admin with the name "all" and the destionation "*" but the postfix log said "user unknown".

BTW, I'm running iRedMail 0.5.0-rc1 with mySQL.