making mutt useful offline

i've been using mutt as my mua for over 8 years now. long ago i would ssh to my server and run it on local maildirs, but as soon as i started using smartphones and multiple computers i had to switch to an imap+ssl setup. mutt's header_cache option has long made accessing large mailboxes snappy, and the recent message_cachedir option available in 1.5 makes browsing through messages with attachments equally snappy over imap.

a useful side effect of message body caching is that it provides an offline copy of entire mailboxes which get synchronized automatically and can easily be read in mutt as a local mailbox... well, almost.

because the message caches are just stored in a directory one-message-per-file and one-mailbox-per-directory, this doesn't match any known mailbox format so mutt will just error out saying it's not a mailbox when trying to read it with mutt -f. however, if those messages are saved to a cur/ subdirectory and a new/ subdirectory is also present, mutt will recognize it as a maildir and read it properly. yes, the files inside the cur/ directory will not have "proper" names with a colon, version number, comma, and flags, but it's enough to get mutt to read them.

i made a patch to mutt 1.5.18 that changes the message cache functions to write to a cur/ subdirectory and ensure that a new/ subdirectory also exists. with these in place and all of your folders cached, an offline muttrc can be created:

 source ~/.muttrc

 unset spoolfile
 unset folder
 unset postponed
 unset record

 # required to prevent imap connections even though spoolfile and folder are changed,
 # for some reason
 set imap_passive=yes

 # point to the appropriate directories
 set spoolfile=~/.mutt-cache/imaps:user@mail.example.com/inbox/
 set folder=~/.mutt-cache/imaps:user@mail.example.com/

and then mutt can be invoked as (or an offline_mutt shell alias set to):

mutt -R -F ~/.muttrc.offline

which will provide read-only views of all of your mailboxes and retain all of the other macros and searching, filtering, and other functions you already have configured in mutt. note: not using -R may change message flags which will rename the cache files and cause them to get out of sync.