There are multiple reasons an email sent to your site inbox will not get converted to a ticket. We will add possible reasons to this article as we encounter them.
From address the same as internal address
We discerned this by going to the email logs to find the message that did not show up anywhere else, then checked the message details. This was the error message it contained.
tried to find account from basic to/from data, got 45758 tried to find account from message type data, got 45758 tried to find records: customer-173528657 contact-892931 invoice- ticket- estimate- EmailLoopPrevention triggered, email received from 'firstname.lastname@example.org' was detected as also a sender we use.!
The error basically indicated it was preventing an endless email loop from being created. In looking at the email the user had sent from an automated system, the "From" address—email@example.com—was the same as the "To" address. That is, it was one of the mailboxes used on their site found in Admin > Emails - Mailboxes (inbound email).
The From address cannot be the same as one of the addresses used in your Mailboxes page.
You also should not use any admin user emails in customer records, nor should they be the same as any of your mailboxes.