Use your production.log of the day the issue occured and lookup the message id.
If it hits your instance then you will be able to find it within the log and will also be able to see what happened to it.
As I said, the classics are:
- no permissions in the affected group
- another instance is pulling the mail (e.g. test instances)
- (doesn’t match your case but for completeness) Zammad is unable to process the mail and puts it to
tmp/unprocessible_mail/
Zammad will never generate a answer mail or agent notification unless the ticket has been created successfully. Never ever.