add minimal queue administration docs

describe the buttons and their functions with the most common usage.

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>

pmg-administration.adoc

@@ -178,4 +178,38 @@ the status is shown as 'queued/accepted' instead of 'accepted/<status>'.
 Postfix Queue Administration
 ----------------------------
 
-TODO
+
+Mail-queues are one of the central concepts of the SMTP protocol. Once a
+mailserver accepts a mail for further processing it saves it to a queue.
+After the mail is either relayed to another system, stored locally
+or discarded it is deleted from the local mail-queue.
+
+If immediate processing is not possible, for example because a downstream
+mailserver is not reachable, the mail remains on the queue for later
+processing.
+
+The Queue Administration view provides a succinct overview, as generated by
+the 'qshape (1)' utility, about the current state of the postfix mail-queue.
+
+It shows mails for which domains were not delivered, and how long they have
+been queued.
+
+The three Action Buttons on top provide the most common queue operations:
+
+'Flush Queue'::
+
+Attempt to deliver all currently queued mail, for example if a downstream
+server has become available again.
+
+'Delete All Messages'::
+
+Delete all currently queued mail, for example if the queue contains only spam.
+Attempt to deliver all currently queued mail.
+
+'Discard address verification database'::
+
+Clear the recipient verification cache.
+
+
+In the 'Deferred Mail' tab you can filter and examine deferred mail in more
+detail by looking at the headers.