If you have marked the sender or their domain as Junk in eM Client, go to Menu > Settings > Mail > Privacy > Blacklist > Manage Blacklist. See if the sender’s address or domain is in the list and remove it.
That is something you will need to discuss with your email provider.
We believe that Spam should be filtered directly on the server so eMClient includes only one default Spam rule that moves messages marked by SpamAssassin as spam to the Junk folder.