Constant growth of Exim Mail queue?

Question

linux

It all started when I installed the Munin monitoring system on my initially personal and experimental server.
On the server, in addition to my sites, there is a forum for a small gaming community on phpBB. After a while, I noticed that the "exim mailqueue" parameter in Munin was growing. After a while, Munin highlighted it in red, indicating that it was critical.
At the same time, absolutely nothing terrible happened. Mailings from the forum were still being delivered and are being delivered, registration letters are coming in (everyone except for urk.net users, but this, in my opinion, is not very relevant to the question).
Actually, here is the annual chart for this parameter (munin is set in January):

I have a suspicion that the growth of the queue is caused precisely by the forum, where there are regular mailings to users of the same group.
I would like to ask if I can be right, because of what the queue of letters can grow and why these letters get into the queue and not sent?

Answer 1

Most likely, several people have problems with mail. The letter is put in hold and tries to deliver it for a long time (and the server does not reject it, but refuse, that is, “not ready to process right now”). Make a queue output, see which addresses are in the top.

Answer 2

what can cause the queue of letters to grow and why these letters get into the queue, but are not sent?

Have you tried to view several messages (-Mvc) from the queue (-bp) and their delivery logs (-Mvl)?