How can I remove the label Nonjunk

[Olivia_Rust]

eM Client does not show a tag NonJunk or any other tag based on the header X-Spam-Flag: NO.

Have you perhaps used Thunderbird (or another mail app based on Thunderbird) previously? Their adaptive Junk filter is what assigns the NonJunk state as a tag and then they keep that tag hidden - but when synchronized to another app via IMAP, it shows as any other custom tag.
So turning off that feature in Thunderbird and then removing the tag manually is what solves the issue.

Another user on the forum provides great advice here:

Mass-removal of "NonJunk" tag or disabling this features?

I know this is an old thread, but wanted to reply on here for anyone who might be searching. The NonJunk tag is indeed being added by Thunderbird’s adaptive spam filtering. It can be disabled in [Thunderbird] Account Settings > Junk Mail > Uncheck Adaptive Spam Filtering.