I'm having an issue where a clients transational emails (i.e. an email sent after someone purchases a product) are ending up in spam folders.

All my usual checks (image balance, language, subject lines, bad lists - which I don't think exist since it's triggered email not from a list) haven't turned anything up. I havem't edited anything yet so It ha a lot of default/slightly modified code from the their marketing platform (Magento) but nothing on the surface looks like it should be causing deliverability issues.

Does anyone know what might be causing transactional emails to go to spam?