What criteria does Apple email use to classify mail as "JUNK"?

Hi marty226, thank you for posting in Apple Support Communities.

Apple iCloud uses trend analysis, dynamic lists and other technology to automatically detect and block junk mail before it reaches your inbox. But there is also the option for you to manage junk emails as well and the following link explains in more detail how to do that:

https://support.apple.com/en-au/HT202315

I did that happen to me quite regularly for only one user which confused me because i wasn’t expecting to look in my Junk mail and the only way I resolved it was to move it from Junk into my Inbox and I think after moving two, they all ended up staying in my inbox. But I agree it was random.