> It's probably a bug, but it must have something to do with your styles, as
> I don't get this.
You are right.  When I start zsh without my .zshrc it works as expected.

> Have you looked at the output from _complete_debug (use <ctrl-x ?> instead
> of <TAB>)?
I did just now (didn't know this feature till now), but I must admit, that
I don't understand the output.  I'll attach it to this mail for the
experts.  I assume the relevant part is here:

 trace _all_labels:39(while else)> -M m:{A-Za-z}={a-zA-Z} -F _comp_ignore -J files -X mailbox specification files expl mailbox specification
 _all_labels:39: command not found: -M

This looks like it has to do with this zstyle option of mine:
 # complete case insensitive
 zstyle ":completion:*" matcher-list 'm:{A-Za-z}={a-zA-Z}'

But if I remove that, this happens:
 % mutt -f <TAB>
 % mutt -f _all_labels:39: command not found: -F
Not much better. :-)

Here are all my zstyle options that concern mutt:
 muttaliases=( `/home/spiegl/bin/list.mail.aliases` )
 muttusers=( `cat $HOME/.br.users` $muttaliases )
 zstyle ':completion:*:mutt:*' users $muttusers
 # where to find mutt "mailboxes" and "alias" settings
 muttrc=~/.mutt/local
 # if no completion can be found, show all files
 zstyle ':completion:*:*:muttf:argument*' tag-order globbed-files all-files

And in ~/.mutt/local is (not only) this:
 # Mailboxes
 mailboxes =alert
 mailboxes =bugtraq =tripwire
 mailboxes ! =out/andy =out/heidi
 mailboxes ~/News/my.articles +postponed
 mailboxes =root =weberror =wwwcron =postmaster =webmaster =backupmaster
 mailboxes =ard-webtechnik =brnet
 mailboxes =junk =junk.spiegl =orakel
 mailboxes =sub/cus/rundbriefe =sub/cus/rundbriefe.muc
 mailboxes =sub/mutt =sub/zsh
 mailboxes =sub/linux/linux-ug =sub/linux/debian.security =sub/linux/debian.announce
 mailboxes =sub/linux/debian.commercial =sub/linux/debian.user.de =sub/linux/debian.user
 mailboxes =sub/kira =sub/akte


While we are talking about mutt completion, I'd really like to program it
so that I can use '=' at the front of a mailbox name to abbreviate ~/Mail/, 
but all my tries so far didn't work. :-(
In tcsh it was done like this:
complete mutt       c/-/"(a b c f F H i m n p R s v x y z Z h)"/ c/*@/\$hosts/ \
            c@=@F:$HOME/Mail/@ c@+@F:$HOME/Mail/@ \
            C@[./]@f@ n/-s/x:'<subject>'/ \
            n/-a/f/ n/-f/f/ n/-F/f/ n/-H/f/ n/-i/f/ \
            n@*@'`cat $HOME/.br.users;$HOME/bin/list.mail.aliases`'@

Thanks,
 Andy.

PS: I am using zsh 3.1.9-dev1-debian0619 (by Clint Adams <schizo@debian.org>)
-- 
 E-Mail: Andy@spiegl.de     URL: http://andy.spiegl.de
 PGP/GPG: see headers
                                o      _     _         _
  --------- __o       __o      /\_   _ \\o  (_)\__/o  (_)
  ------- _`\<,_    _`\<,_    _>(_) (_)/<_    \_| \   _|/' \/
  ------ (_)/ (_)  (_)/ (_)  (_)        (_)   (_)    (_)'  _\o_
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 Plug-and-Play is really nice, unfortunately it only works 50% of the time.
 To be specific the "Plug" almost always works.            --unknown source