- we need permanent storage for UIDs. with mbox use X-UID like UW-IMAP,
   with maildir a) file:2,flags,Uuid b) file,U=uid:2,flags. uid validity
   would be in .uidvalidity file. the b-case would require that to be done
   by the client moving it from new/ to cur/

 - DELETE/RENAME: when someone else had the mailbox open, we should
   disconnect it (when stat() fails with ENOENT while syncing).

 - bugs
    - maildir: if mail file isn't found, it may be because it was renamed
      (flag changed). we must then sync the directory and see again if the mail
      is found
    - SIGHUP didn't update imap_listen. this is a bit annoying to fix though,
      since new listen() may fail for a few times because login processes may
      not die immediately..
    - SIGHUP doesn't update log file location.
    - CREATE a, CREATE b, save mails into them, DELETE a, RENAME b a.
      -> breaks if a+b have same UIDVALIDITY. We could update the
      UIDVALIDITY for the renamed mailbox and all mailboxes under it.
      Then return with tagged "OK [NEW-UIDVALIDITY 1234] Renamed". Assuming
      other IMAP people agree to that.

 - reliability fixes:
    - maildir: check if there's base name conflicts when syncing. the hash
      could also be built against index.data so it'd take less memory
    - if we deleted mail from index but didn't write modify log, other
      dovecots don't handle it properly. they either assert at index-sync.c:42
      or if new mails have also been added since, they don't notice it at all
      actually, that breaks reads as well since we get expunges only from
      the old file.. and check that deleting file does "inconsistency error"
    - if imap process notices that both modify logs are getting full because
      it's client isn't syncing, the client should be disconnected
    - we don't handle out of memory conditions too well, malloc failing kills
      the process which is good enough (and likely never happens), but mmap()
      failures aren't handled too well. Rather should be handled in similiar
      way to locking failures, so that at least we don't try to rebuild the
      index because of it.
    - limit folder hierarchy levels? user can now create eg. a/a/a/a/...
      and then start renaming them from end to beginning, which probably will
      at some point start causing syscall failures which will fill up logs.
    - fsck should check binary tree

 - mbox
    - message_parse_header() skips headers (even multiline) larger than max.
      input buffer size, which in our case means that headers larger than
      8192 bytes will be silently skipped when saving mails.
    - Optimize rewrites by using empty spaces after X-Keywords
    - Move data within file instead of writing it to temp file. We can now do
      moving easily with o_stream_send_istream().
    - if a file isn't valid mbox and it's tried to be opened, say it in one
      line in error log, not 6..
    - When expunging the first message we could move the X-IMAPbase header
      to next message to avoid full rewriting later.
    - We shouldn't send X-IMAPbase, Status, X-Status, X-Keywords, X-UID, etc.
      headers to client - they may change and clients must see messages as
      immutable. Create istream-filter and use it.
    - two adjacent From-lines breaks us. not too easy to fix though.
    - Handle UW imapd's "DON'T DELETE THIS MESSAGE" message.

 - checks:
   - if we have entries in modifylog with UID 10..11, 9..12, 8..13 etc.
     do they work correctly?
   - make sure SELECT rebuilds index properly when next_uid is near 32bit value
   - make sure connection limits work
   - check that both header and envelope address writer produce same output
   - check if some asserts should be changed into if/i_panic code, so that
     disabling asserts would be possible

 - enhancements:
    - option to disable SORT, SEARCH and other memory/cpu-intensive features.
      defaults and per-user by dovecot-auth.
    - when fetching body/envelope/etc we could try to cache it immediately if
      we can get lock with try_lock.
    - maildir could support also the dirty-flag in messages. files would be
      renamed "whenever there's time" (that'd require the indexer program, or
      forking and doing it in background)
    - optionally keep the message file name as it's UID. Then we don't have to
      save the filename anywhere.
    - send EXISTS immediately after new mail arrives.
        - linux: we can use dnotify for maildir (but not mbox I think, we'd
	  get interrupted all the time if we checked eg. large /var/spool/mail)
	- *bsd: kqueue() can notify changes in mbox and maildir
    - .subscriptions and .customflags needs nfs-safe locking
    - OpenSSL: support generated DH parameters
    - SSL: Support password protected key files. Support reading the password
      from user at runtime (dovecot startssl or something).
    - multiline headers can cause our memory usage to go up. that should be
      fixed somehow. try to change things to be able to handle one line at a
      time? Well, other IMAP servers have same problem - post 1.0 problem.
    - check with strace what dovecot does when evolution checks new mail,
      it's quite a lot. some things probably wouldn't need to be done
      (mkdirs/symlinking inbox) and other things could be cached in memory.
    - sort: we could create alternative binary tree file(s) for different sort
      conditions, ".tree-sort" or something. sort code itself already supports
      this optimization.
    - tree file: should we instead use b+-tree or something similiar? or at
      least try to do some defragmentation with it, so that the root nodes
      would be kept at the beginning of the file.
    - use vsftpd-like safebufs, ie. keep non-rwx page before and after the
      memory we want to use.
        - mmap_anon()
        - mmap()ing files would probably need to first go through anon_mmap()
	  and then use MAP_FIXED. annoying that it slows the mmaping..
	- data stack should use mmap_anon()
    - option: copy /var/mail/$user to INBOX when logged in. nice for not missing
      any mails with quota enabled
    - support zlib compressed mbox/maildir? mbox maybe just read-only.
    - THREAD=ORDEREDSUBJECT - although pretty useless I'd think.
    - sql authentication and mail database support. create some generic lib-sql
      which both can use
    - maildir: change it to use '/' as hierarchy separator to allow '.'
      characters (for usernames in shared folders)

 - logging
    - Login: username 1.2.3.4:1025 5.6.7.8:993 imaps,compressed
    - Logout: username 1.2.3.4:1025 5.6.7.8:993 imaps,compressed in:1000 out:1000000
    - n failed login attepts (before failure or success, once in n seconds)

 - lib-charset:
     - utf8_toupper() is a must. and a bit difficult if we want to do it right.
     - add support for other things than iconv() as well? we could reuse
       the code from cyrus or courier
     - cache iconvs? they'd probably be faster if we just reset the
       conversion instead of opening new one every time. and there will likely
       be only one or two charsets which are used for nearly all conversions.

 - should we allow following symlinks in mbox/maildirs? they are now.
    - if we implement shared mailboxes with shared indexes, never do that or
      others could symlink your personal mailboxes and see the indexes
      created for it which may contain envelope etc. data
    - this allows circular mailbox hierarchies which should be prevented by
      eg. allowing max. 20 hierarchies.

 - index:
    - we could try compressing same from/to/subject fields into a single
      location in data file. requires larger changes..
    - read-only support for mailboxes where we don't have write-access
    - we should try to avoid completely rebuilding indexes unless they're
      corrupted. especially if we later want to support some read-only boxes
      and keep the mail flags only in index file. fsck() could verify that
      records are ok, and that if data file isn't ok the record is deleted.
    - if .customflags is removed and Maildir files have custom flags, add
      "unknown1" "unknown2" etc. flags to .customflags file for each found flag
    - skipping deleted records in .imap.index would be faster if we saved the
      deleted block size to first/last record, so we could just jump over them.
    - support storing message headers into indexes. this could be useful when
      indexes are in local disk but actual mails are accessed through NFS
    - we could send flag changes after all commands by making expunge/flags
      sync counters separate for modify log. flags would need to update the
      seq though, too slow?
    - if we wanted to support huge mailboxes with small memory usage, it'd now
      be possible if we just instead of mmap()ing the whole index files would
      have maybe 3-4 256k mmap()ed areas which we move based on the need.
	- should work fine with .imap.index and .imap.index.data
	- log files aren't affected by mailbox size
	- if the tree file also kept constantly moving the nodes so that
	  tree's root was at the beginning of the file, we could use this mmap
	  caching with it too
	- but, is it worth the trouble really? the OS can do all this itself,
	  only thing we're doing is keeping the processes virtual memory usage
	  small.

 - SEARCH:
    - message_body_search() could accept multiple search keywords so we
      wouldn't need to call it separately for each one (so we wouldn't need
      to parse the message multiple times).
    - message_body_search() could support NULL MessagePart and the searching
      could be done while parsing the message. this would need changes to
      message_parse() as well.
    - could optionally support scanning inside file attachments and use
      plugins to extract text out of them (word, excel, pdf, etc. etc.)
    - use a trie index for fast text searching, like cyrus squat?
    - Create our own extension: When searching with TEXT/BODY, return
      the message text surrounding the keywords just like web search engines
      do. like: SEARCH X-PRINT-MATCHES TEXT "hello" -> * SEARCH 1 "He said:
      Hello world!" 2 "Hello, I'm ...". This would be especially useful with
      the above attachment scanning.

 - general:
    - sieve (rfc3028), we can use Cyrus Sieve
    - rfc2231 continuation support
    - rfc2557 support for BODYSTRUCTURE, as specified by latest IMAP4rev1 draft
    - lmtp server
    - create indexer binary
    - support Maildir++ quota
    - provide some helper binary to save new mail into mailboxes with CR+LF
      line breaks?
    - some kind of IMAP proxy for load distributing
    - maybe give more untagged NO/ALERT replies? like when mailbox is in
      inconsistent state. and when UIDs are reordered because they're too
      large.
    - imap/ and lib-imap/ should allow infinite number of custom flags, it's
      storage's problem if it can't handle too many of them.
    - things calling message_send() could verify that it wrote enough data.
      if not, fill the rest with spaces and return failure. -1 = error,
      0 = filled, 1 = ok.

 - cleanups:
    - check if t_push()/t_pop() should be added somewhere
    - try to fix @UNSAFE code to use buffer API instead
	- subscription-file.c, custom_flags, ioloop
	- [io]stream-file.c?
    - grep for FIXME
    - index/create_temp_file is used in only two places. once mbox rewriting
      doesn't need it, get rid of it.

 - auth / login:
    - kchuid, SRP, anonymous SASL
    - Digest-MD5: support integrity protection, and maybe crypting. Do it
      through login process like SSL is done?
    - for invalid user/pass, wait for a while before giving a reply to user
    - dovecot-auth should limit how fast authentication requests are allowed from
      login processes. especially if there's one login/connection the speed
      should be something like once/sec. also limit how fast to accept new
      connections.

capabilities:
 - preferrably all should be possible to #ifdef away by a configure
   option (--without-capabilities=acl,namespace,...)
 - possibility to disable them from config file
 - acl (rfc2086, draft-ietf-imapext-acl), namespace (rfc2342)
     - probably do it like cyrus. "user.<username>" to access other
       users, with "" defaulting to "user.<myself>". these should be
       configurable however.
     - shared namespaces? maybe configurable in config file
     - easiest way to do ACL would be to use unix modes, but is that
       useful at all? Well, ACL2 has a bit better support for that, so
       maybe we could support it.
     - otherwise gets a bit trickly, we could keep all mail in "imapmail"
       group and 0600/0700 mode by default, but when mail is shared to others,
       the group read/write access bits would be set. or alternatively we
       could launch another imap process to handle it, which we should support
       anyway. ACLs could be stored into ".acl" ascii file in each folder.
     - support for private and shared flags, configurable by mailbox admin.
       this isn't in any draft yet, but ACL2 author was going to create one.
       [SHAREDFLAGS (...)] would specify which ones are shared, don't know yet
       how they would be configured.
 - quota (rfc2087, draft-cridland-imap-quota)
     - give filesystem values only to admins
     - support for Maildir++, probably no need to support more.
       quota capability supports complex quota configuration, but if
       no mailer supports them we probably shouldn't bother either
 - id (rfc2971)
     - must be configurable what gets sent, default to only name=Dovecot
     - separate pre/post-login settings
     - optionally log configured parts of the client information, but only
       once, probably at the same time as logging "Logged in",
       "Disconnected", etc.
     - remember to force truncating values longer than 30 chars,
       especially before logging
 - mailbox-referrals (rfc2193)
     - this is useful whenever we would otherwise need to make the
       connection ourself. for example load balancing and shared mailboxes
       requiring another UID to run.
     - this rfc defines no exact way for server to detect if client
       supports referrals or not. I don't think there's much point in
       supporting only referrals, as most clients don't support them.
       Instead we should return referrals when we know that client
       supports them, otherwise do the connecting ourself. If client
       issues RLIST or RLSUB command, it's safe to assume it supports
       referrals.
     - for load balancing this works just fine, but what about shared
       mailboxes which require different UID? If we login with our own
       username, we end up with our own UID instead of what we wanted.
       IMAP URLs don't support separated authorization id which would
       have made this very easy.. We could give the "userid@group" as
       userid, but clients probably treat it as different userid and
       ask the password again.
     - problems, problems, .. maybe not worth the trouble.
 - uidplus (rfc2359)
     - uid expunge: no problem
     - append, copy: oh no. these would slow down things and make
       handling them much more difficult. currently we just store the
       mails to destination mailbox without touching the indexes. since
       we'd need to know their final UID, we'd have to lock the indexes
       and mbox) fsck() first and append() next to find out the uid,
       maildir) move the mail directly into cur/ and index it.
     - except, the index updating is faster than requiring to sync the
       changed mailbox. we'll probably create such deliver-binary anyway,
       so it can't be that difficult to code.
 - drafts:
     - http://www.imc.org/ids.html
     - annotate (draft-ietf-imapext-annotate)
	 - per-message annotations. this will be major change. especially
	   because currently there's no suitable storage for them, and
	   they'll probably change all the time.. maybe if we moved into
	   berkeley db to store the .data file and these annotations.
	 - this is separate problem from index files. indexes are treated as
	   temporary files, annotations are permanent data. we'd have to
	   support non-db way to do this too, which would probably be just a
	   simple (slow) text file.
     - annotatemore (draft-daboo-imap-annotatemore)
	 - server and per-mailbox annotations. much easier than
	   per-message annotations, but they'd be easier to place into
	   db as well.
     - binary (draft-nerenberg-imap-binary)
	 - perhaps not too useful. I'd like to make Dovecot fully
	   binary-safe though.
     - view (draft-ietf-imapext-view)
         - slow, complex, luckily draft expired almost two years ago.
	   i hope i don't have to implement this :)
	 - can be done client-side just fine (evolution's virtual folders)