Dovecot 2.4

So I did my regular Debian Unstable upgrade yesterday, and the version
of Dovecot IMAP is now 2.4, with a config file format that is not
quite backward-compatible.

That’s OK, I soon figured out what changes to make to get it to
continue working with my existing maildir setup
<https://doc.dovecot.org/main/core/config/mail_location.html#mail-location-setting>
<https://doc.dovecot.org/main/core/config/mailbox_formats/maildir.html>.
You tell it you want to use maildir with

    mail_driver = maildir

but now (not sure if this was true before) you have a choice of two
maildir layouts, specifiable with the “mailbox_list_layout” keyword.
The “maildir++” option corresponds to what I currently have, but the
other option, “fs”, looks interesting.

The thing with “maildir++” is you have dotfiles within dotfiles, which
always struck me as being a bit pointless. The “fs” option avoids
this, and uses a subdirectory hierarchy to reflect your mail folder
hierarchy.