Troubleshooting permission denied emails?

Can anyone offer any guidance to troubleshooting permission denied
emails? Is there a way to see what triggered the denial?

Some of our queues have incoming email addresses where our customers
send us messages. Other queues are for internal use only by our group.
We create project tickets in those queues, and sometimes transfer
incoming emails to these queues.

In all of our queues (whether they have incoming email addresses or not)
we have ReplyToTicket and CreateTicket rights assigned to ‘Everyone’

In this particular case a ticket was created in a queue that does not
accept email, but ‘Everyone’ has rights to create or reply to tickets.

After creating the ticket, a sysadmin in our group replied to the ticket
and CC’ed a user in that reply (note, not a ticket CC, but just a CC to
someone on the reply).

When the person who got the CC replied, she then received a permission
denied message.

I don’t understand why this would be since 'Everyone" can create or
reply to tickets in this queue.

I checked, and no RT user was created based on the person emailing in.
The message just got rejected.