4 June 2019
Go to File > Account Settings > Account Settings
Click on your email account. Then click the “Change” button
Click change, click on the “More Setting”
Click on Advanced, there you can see your Mailboxes. On the “Open these additional Mailboxes:” select your mailboxes (in my case I have already added) then click Add. After that click the Apply button, then click the OK. Then just click Next, then finish.