The mailbox format used is called maildir, and mail will reside inside a directory called
Maildir inside your home directory. The mail software provided understands this format. This is a directory, not a single file, so you can't edit it or view it with a text editor. Always use one of the recommended methods to access your mail.
A maildir format directory can contain any number of folders inside it, so you are not limited to just one mail folder. All mail folders will however reside inside that one directory called
It's easy to download mail in bulk using Roundcube. After logging into Roundcube, first select the messages to be downloaded. Then look for a horizontal three-dot menu (⋯) probably near the top right of you screen. Click on this and follow the menus.
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