In order for
mu (and, by extension,
mu4e) to work, you need to
have your e-mail messages stored in a
maildir15 — a specific
directory structure with one-file-per-message.
If you are already using a maildir, you are lucky. If not, some setup is required:
isyncto download your messages into a maildir (~/Maildir, often). Because it is such a common case, there is a full example of setting
offlineimapand Gmail; see Gmail configuration.
qmail), you can teach them to deliver into a maildir as well, maybe in combination with
procmail. A bit of googling should be able to provide you with the details.
mu only supports a single Maildir, it can be spread across
different file-systems; and symbolic links are supported.