Could be that address or email domain has been accidentally added in the blacklist and so then automatically going to the spam / junk folder.
See @gary post on how remove email addresses from the blacklist in eM Client V9 or V10.
If it’s not in the Blacklist then go into eM Client rules via “Menu / Rules” (pc) or “Tools / Rules” (Mac). Then “dblclick all the rules in there” (except the Spam rule) to see what you have setup within each rule as could be a local causing it.
Also along the top in eM Client rules you can select “Server Rules” in the dropdown on the right of Local Rules, where depending on your type of mail account can also sometimes show you any server rules that you may have setup already. So check in there as well just incase of a server rule.
Note:- For troubleshooting if there is any eM Client local rule issue and you have too many rules to check, you could also eg: just “uncheck all the rules” and then Save and Close Rules and test..
Now if there is no blacklist or rule setup causing it, then it will more than likely “just be your Server Mailbox auto detect spam engine” moving those emails directly to the junkmail folder from the Inbox when they arrive which in-turn will then show / mirror in the eM Client junkmail folder.
So next go online to your junkmail folder and "mark those senders emails as Not Spam or Not Junk” which then your server mailbox will learn from then on to keep those emails in your Inbox.
Note: If those emails then keep going directly into your Junkmail folder online, you would then contact your mailbox technical support for assistance as will then be mailbox related.