X-ME-VSCategory is a Fastmail (Messaging Engine) proprietary header which can be used in filters to automatically categorise incoming messages.
It appears to be added to messages received after 20 March 2018, but is only documented by Fastmail as “a category as determined by our content filter.” (The email delivery process § What headers are added by Fastmail?)
| X-ME-VSCategory |
|---|
| clean |
| commercial:mce |
| transactional:account |
| transactional:alerts |
| transactional:purchases |
| transactional:travel |
| phishing |
* transactional:travel noted by @faisal@social.lol