A little tip about the postfix plugin.
If you have a Debian Bullseye server, where the postfix plugin does not work, so no mailq graphic is displayed.
Then please check if there is an entry
authorized_mailq_users = nagios, icinga, netdata
in /etc/postfix/main.cf.
Hi, @DarkDau. Thanks for the info! Consider making a PR with postfix readme update 
So postfix doesn’t care about /etc/group entries.
it decline everybody who is not expliciet declared in min.cf
As non native English speaker. What is PR? Problem Report or what?
I will not make a Pull request.
I fear that would more confuse than help.
So I describe the issue here more in detail.
On the “old” Debian Buster server everything worked out of the box. mailq works.
(postfix 3.4.14)
On Debian Bullseye not. (postfix 3.5.13)
My attempts to make the mailq visible led to nothing. Until I came to the proven method to run the plugin as netdata user.
The first result was already visible without the plugin
su -s /bin/bash netdata
/usr/sbin/postqueue -p
postqueue: fatal: User netdata(115) is not allowed to view the mail queue
Clear case, I thought netdata is not in the group of postfix.
netdata added, retested as netdata… unsuccessful.
The problem is, with Bullseye the rights for other users are defined by Postfix in the main.cf.
authorized_mailq_users = nagios, icinga, netdata
So one could make a simple test if netdata is already there:
~#grep authorized_mailq_users /etc/postfix/main.cf
it should contain netdata
To fix add netdata in main.cf and restart postfix
service postfix restart
@DarkDau I checked the postfix docs and
authorized_mailq_users (default: static:anyone)
List of users who are authorized to view the queue.
By default, all users are allowed to view the queue.
So you had non-default (with users) authorized_mailq_users before upgrading?
On the old Debian it was not used.
My bullseye was a new installation, different hardware. could be that I set it up, can’t remember 
My quick search in the docs I used I did nnot find it
Ok, I made a PR with the Postfix readme update - docs(postfix): add a note about `authorized_mailq_users` by ilyam8 · Pull Request #13515 · netdata/netdata · GitHub