In order for mu (and, by extension, mu4e) to work, you need to have your
e-mail messages stored in a
Maildir; in this manual we use the
term ‘maildir’ for both the standard and the hierarchy of maildirs that store
your messages — 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:
getmail,
fetchmail, offlineimap or isync to download your messages
into a maildir (~/Maildir, often). Because it is such a common
case, there is a full example of setting mu4e up with
offlineimap and Gmail; see Gmail configuration.
postfix or 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.
While a mu only supports a single Maildir, it can be spread across
different file-systems; and symbolic links are supported.