Cargo Features

Maildir has no features set by default.

[dependencies]
maildir = { version = "0.6.4", features = ["mmap"] }
mmap = memmap2

Features from optional dependencies

In crates that don't use the dep: syntax, optional dependencies automatically become Cargo features. These features may have been created by mistake, and this functionality may be removed in the future.

memmap2 mmap?

Enables memmap2 ^0.5.8