Log into Webmail using Horde. Example: https://webmail.yourdomain.com
Warning: If you log into Roundcube webmail, you’ll need to click your email address in the upper corner, then Horde.
or under cPanel Webmail select horde
Right-click Inbox and click Export emails in Inbox, Drafts, Sent, and their subfolders.
Choose to download a standard or Zip compressed MBOX file and click OK.
Save the file.