In summary: make sure to add ‘email@example.com’ to your ‘never treat as spam’ rules. Thank you.
Often wanted email messages go straight to a spam/junk folder as a result of an inaccurate spam detection algorithm and the recipient does not get to see the wanted message. To remedy this one must 'train' the detection engine, and then go back periodically to confirm it has not itself been modified and become over-zealous.
All email users have an account on a server (their repository) such as Gmail, Hotmail, or AOL perhaps. Spam detection usually occurs there and that must be taught when a mistake has been made - most commonly by right clicking a message and choosing to 'mark it as not spam'. Note that simply moving a message to your Inbox will have no long term effect.
To avoid being caught by modifications to the spam detection engine you may like to create a filter to override that.
How this is done differs with each provider but using Gmail as an example go to
https://mail.google.com/mail/u/0/?ogbl#settings/filters and click the Create a new filter link. Fill in the From address appropriately and click the Create filter link. In the following pop-up window check Never send it to Spam and also Also apply filter to matching messages then click Create filter.
If email access is done using the provider's web interface accessed via a browser (e.g. Chrome, Edge, Firefox, Safari) then that may be all that needs be done. But many people find the use of a mail client (e.g. Outlook, Thunderbird, Apple Mail) running on their own computer to be a much better way of handling messages. In this case a second spam detection process is at work and needs be trained.
If you use the Outlook mail client then select a message from the person to be disallowed from spam and from the Home tab, Delete group click Junk then Never Block Sender. You may also benefit from clicking Junk then Junk E-mail Options... Click the Safe Senders tab and check the Also trust email from my Contacts and Automatically add people I email to the Safe Senders List.
If you use the Thunderbird mail client then you can create a filter that is processed after the spam detection engine completes. From the Tools menu choose Message Filters. Click the New... button then think of an informative name for your creation. Now create a match rule where From begins with Eugene (or whatever sender is being favoured). Now move to the Perform these actions box and select Move Message to then select the appropriate destination (Inbox perhaps). Now click the + sign to add another criterion; at the end of the first pull down menu select Stop Filter Execution. Finally look to the top of the window and check Getting New Mail and select Filter before Junk Classification. OK then close the pop-up window.
The above suggestions will no doubt change with different versions of the software. Also, many other clients and repositories are available and each will have their own way of accomplishing these tasks.