How can I prevent email from a particular sender from being sent to the junk folder
Could be you have either inadvertently added “a rule to automatically move that senders emails to the Junkmail folder, or possibly inadvertently added that senders emails to the blacklist and so also automatically going to the junkmail folder, or it could be your mail server is automatically moving it to the junk folder due to something in the email.
So first go to “Menu / Rules” (Pc) or “Menu / Tools / Rules” (Mac) and you can “dbl-click on any custom rules” (apart from the basic spam rule at the top) to view / check them if you have added custom rules. You can also dbl-click the blacklist in rules if it’s showing in there up to eM Client V9.
If you have eM Client V10, the blacklist is via “Menu / Settings (Preferences) / Mail / Blacklist & Whitelist” and click “Manage Blacklist” at the bottom of the window to see if they are in there.
Now if there is no eM Client local rules setup with that senders email address or domain name and no blacklist setup with their email address or domain, then it will be more than likely your mailbox on the server end automatically moving them either “due to a server spam engine” detecting something in the header, subject or body, or a “server mailbox rule or filter” automatically moving them.
So you would next go into your mailbox online junkmail folder and “mark those emails as not spam / not junk”. That will then automatically move them back to your online inbox and teach your online mailbox to keep them in the inbox for future.
Also check in your online mailbox rules or filters section “for any automatic rules setup to move emails to the spam folder”. As could possibly be there is rules there that have been inadvertently setup online to move that sender to junk mail.
You can also view any server side rules in eM Client (if your server is supported) by clicking the dropdown on the right of Local Rules.