Email plus addressing does not work in Roundcube

Symptoms

When you receive an email using "plus addressing," you cannot find the email anywhere in Roundcube.

Description 

When delivering mail that uses "plus addressing," Roundcube will place the mail into a folder of its own. By default, these folders are not visible in the Webmail client. More information about Plus addressing can be found here:

cPanel Docs - Plus Addressing

Resolution

First, verify that plus addressing is enabled for your email account with the following steps:

1. Login to the cPanel account that contains your email account

2. Click on the Email Accounts icon under the Email section

3. Click on the "Manage" button for your email account
4. Verify that the following option is currently set to "On"
   

   Automatically Create Folders for Plus Addressing

   Please note that this option is enabled by default on account creation but can be manually toggled to off.

Once you have verified that plus addressing is turned on, you must subscribe to the folders to show them in webmail:

1. Login to RoundCube webmail for your email account
2. In the left sidebar, click on the Settings button
3. Click on the Folders section
4. Toggle on the plus address folders that have been created