First release.
A very small PrestaShop module that gives every contact-form message its own reference ID in the e-mail subject, so messages from different customers stop collapsing into one mail thread.
Install: download contactformref-1.0.0.zip below and upload it in Back office > Modules > Upload a module. The ZIP already has the correct contactformref top-level folder, so no renaming is needed.
Note: PrestaShop's contact-form e-mails are off in a stock shop. Turn on Receive customers' messages by email in Modules > Module Manager > Contact form > Configure, otherwise there is no e-mail for this module to act on.
Three ID formats: sequential number, letters in the shape of a PrestaShop order reference, or random digits. Requires PrestaShop 1.7.6+ and PHP 7.0+.