One of my customers used email (a gmail account, no less) as a message queue between their front end site and the back-office processor. This worked quite well

One of my customers used email (a gmail account, no less) as a message queue between their front end site and the back-office processor. This worked quite well for close to a decade I think. It basically evolved from when applications from their original customer facing site were emailed and manually entered into the back-office system by a human. They were looking to automate this with a minimum of changes to too many of the moving parts at once, so I reformatted the sent email to contain an XML payload so the new back-office automation could read and process it (and in a pinch, a human could still review any problem applications) using Java's mail APIs. Things evolved, the front-end web site got replaced with a Wordpress site, but the email message queue kept working for a long time. In the last year or so it was getting more and more onerous though. Reconciling information between the front-end and the mail box showed not all emails were being delivered, and authentication to gmail was becoming more and more of a burdensome moving target. I just recently replaced the whole thing with an API call made from the back-office to the Wordpress site to access the stored data. (The original site didn't store, just emailed, which was why this was not an option historically.)