Why not use the Junk column and just click the dot for that message in that column? It marks or unmarks that message as Junk.

You can right click the mail Toolbar and select Customize. Then find the junk button in the dialog box that opens. Select it and drag it to the toolbar. It is a smart button that is a Junk button for a non-junk marked message and a Not Junk button for a message already marked as Junk.

Using the Junk column won't allow marking batches of mail. I don't have a "not Junk" option when I go to customize the tool bar.

Try reading the answer again slower.

There is a bug in the "smart" JUNK / NOT JUNK button. Airmail's answer was not quite how it works on my system. The tool bar button doesn't display as NOT JUNK until a user actually clicks on the SORT JUNK column that Airmail mentions. The column's pop up hint says it is for sorting your junk messages and mentions nothing about changing the classification from JUNK to NOT JUNK. Once you click on the flame icon in that column for one message and it re-classifies it and sends it back to your Inbox, it will change the JUNK button in the tool bar to display a NOT JUNK button so you can then highlight multiple messages and re-classify them. (plus remember to go find in your Inbox that message you re-classified by clicking on the icon because it is sitting back there.) This bug (or is it a feature) needs to be fixed and the pop up hint re-worded.

Maybe in your world but not mine.

I added the smart button for Junk to the Mail Toolbar and when I select a message and click the button, which is labeled as Junk at this point, the Junk icon appears in the Junk column. When I go back and select that same message the smart button changes to Not Junk. You do have to deselect the message and then go back to it to see the button change from Junk to Not Junk. Works as described for me using version 24.6 on Windows.