If you have your email set up on an email client, there could be a possibility that not all emails are downloaded to your local drive. This usually happens to emails that are set up as a POP protocol.
In cases such as this, you can still retrieve the specific emails using Roundcube through Webmail. The file will be saved as .eml (e-mail message). Below are the instructions on how to download an email using Roundcube:
Step 1. Log in to Webmail.
You may log in via cPanel or use direct links.
Step 2. Under Open your Inbox, click Open.
You may tick the box for Open my inbox when I log in. You can skip this step if this was already ticked.
Step 3. Choose the email that you wish to download.
Step 4. Click More, and choose Download (.eml).
The file will be saved on your local computer. By default, it will be on your Downloads folder or depending on your folder of choice. When you open the file, it will be readable on any mail programs such as Outlook, Mail for Mac, etc.