Chapter 6. Optional Features

Mutt supports several of optional features which can be enabled or disabled at compile-time by giving the configure script certain arguments. These are listed in the “Optional features” section of the configure --help output.

Which features are enabled or disabled can later be determined from the output of mutt -v. If a compile option starts with “+” it is enabled and disabled if prefixed with “-”. For example, if Mutt was compiled using GnuTLS for encrypted communication instead of OpenSSL, mutt -v would contain:

-USE_SSL_OPENSSL +USE_SSL_GNUTLS

Mutt optionally supports the IMAP, POP3 and SMTP protocols which require to access servers using URLs. The canonical syntax for specifying URLs in Mutt is (an item enclosed in [] means it is optional and may be omitted):

proto[s]://[username[:password]@]server[:port][/path]

proto is the communication protocol: imap for IMAP, pop for POP3 and smtp for SMTP. If “s” for “secure communication” is appended, Mutt will attempt to establish an encrypted communication using SSL or TLS.

Since all protocols supported by Mutt support/require authentication, login credentials may be specified in the URL. This has the advantage that multiple IMAP, POP3 or SMTP servers may be specified (which isn't possible using, for example, $imap_user). The username may contain the “@” symbol being used by many mail systems as part of the login name. The special characters “/” (%2F), “:” (%3A) and “%” (%25) have to be URL-encoded in usernames using the %-notation.

A password can be given, too but is not recommended if the URL is specified in a configuration file on disk.

If no port number is given, Mutt will use the system's default for the given protocol (usually consulting /etc/services).

The optional path is only relevant for IMAP and ignored elsewhere.

pops://host/
imaps://user@host/INBOX/Sent
smtp://user@host:587/

As of version 1.2, Mutt supports browsing mailboxes on an IMAP server. This is mostly the same as the local file browser, with the following differences:

Mutt supports four authentication methods with IMAP servers: SASL, GSSAPI, CRAM-MD5, and LOGIN (there is a patch by Grant Edwards to add NTLM authentication for you poor exchange users out there, but it has yet to be integrated into the main tree). There is also support for the pseudo-protocol ANONYMOUS, which allows you to log in to a public IMAP server without having an account. To use ANONYMOUS, simply make your username blank or “anonymous”.

SASL is a special super-authenticator, which selects among several protocols (including GSSAPI, CRAM-MD5, ANONYMOUS, and DIGEST-MD5) the most secure method available on your host and the server. Using some of these methods (including DIGEST-MD5 and possibly GSSAPI), your entire session will be encrypted and invisible to those teeming network snoops. It is the best option if you have it. To use it, you must have the Cyrus SASL library installed on your system and compile Mutt with the --with-sasl flag.

Mutt will try whichever methods are compiled in and available on the server, in the following order: SASL, ANONYMOUS, GSSAPI, CRAM-MD5, LOGIN.

There are a few variables which control authentication:

Mutt provides optional support for caching message headers for the following types of folders: IMAP, POP, Maildir and MH. Header caching greatly speeds up opening large folders because for remote folders, headers usually only need to be downloaded once. For Maildir and MH, reading the headers from a single file is much faster than looking at possibly thousands of single files (since Maildir and MH use one file per message.)

Header caching can be enabled via the configure script and the --enable-hcache option. It's not turned on by default because external database libraries are required: one of tokyocabinet, kyotocabinet, lmdb, qdbm, gdbm or bdb must be present.

If enabled, $header_cache can be used to either point to a file or a directory. If set to point to a file, one database file for all folders will be used (which may result in lower performance), but one file per folder if it points to a directory.

Both cache methods can be combined using the same directory for storage (and for IMAP/POP even provide meaningful file names) which simplifies manual maintenance tasks.

In addition to caching message headers only, Mutt can also cache whole message bodies. This results in faster display of messages for POP and IMAP folders because messages usually have to be downloaded only once.

For configuration, the variable $message_cachedir must point to a directory. There, Mutt will create a hierarchy of subdirectories named like the account and mailbox path the cache is for.

For using both, header and body caching, $header_cache and $message_cachedir can be safely set to the same value.

In a header or body cache directory, Mutt creates a directory hierarchy named like: proto:user@hostname where proto is either “pop” or “imap.” Within there, for each folder, Mutt stores messages in single files and header caches in files with the “.hcache” extension. All files can be removed as needed if the consumed disk space becomes an issue as Mutt will silently fetch missing items again. Pathnames are always stored in UTF-8 encoding.

For Maildir and MH, the header cache files are named after the MD5 checksum of the path.

Mutt does not (yet) support maintenance features for header cache database files so that files have to be removed in case they grow too big. It depends on the database library used for header caching whether disk space freed by removing messages is re-used.

For body caches, Mutt can keep the local cache in sync with the remote mailbox if the $message_cache_clean variable is set. Cleaning means to remove messages from the cache which are no longer present in the mailbox which only happens when other mail clients or instances of Mutt using a different body cache location delete messages (Mutt itself removes deleted messages from the cache when syncing a mailbox). As cleaning can take a noticeable amount of time, it should not be set in general but only occasionally.

The Sidebar shows a list of all your mailboxes. The list can be turned on and off, it can be themed and the list style can be configured.

Sidebar adds the following functions to Mutt. By default, none of them are bound to keys.

This command specifies mailboxes that will always be displayed in the sidebar, even if $sidebar_new_mail_only is set and the mailbox does not contain new mail.

The “unsidebar_whitelist” command is used to remove a mailbox from the list of whitelisted mailboxes. Use “unsidebar_whitelist *” to remove all mailboxes.

If the sidebar_indicator color isn't set, then the default Mutt indicator color will be used (the color used in the index panel).

The Compressed Folder patch allows Mutt to read mailbox files that are compressed. But it isn't limited to compressed files. It works well with encrypted files, too. In fact, if you can create a program/script to convert to and from your format, then Mutt can read it.

The patch adds three hooks to Mutt: open-hook, close-hook and append-hook. They define commands to: uncompress a file; compress a file; append messages to an already compressed file.

There are some examples of both compressed and encrypted files, later. For now, the documentation will just concentrate on compressed files.

The shell-command must contain two placeholders for filenames: %f and %t. These represent “from” and “to” filenames. These placeholders should be placed inside single-quotes to prevent unintended shell expansions.

If you need the exact string “%f” or “%t” in your command, simply double up the “%” character, e.g. “%%f” or “%%t”.

open-hook regexp shell-command

open-hook '\.gz$' "gzip -cd '%f' > '%t'"

close-hook regexp shell-command

close-hook '\.gz$' "gzip -c '%t' > '%f'"

append-hook regexp shell-command

append-hook '\.gz$' "gzip -c '%t' >> '%f'"