Hello,
like described at wiki "Mailbox settings":
protocol imap { namespace inbox { mailbox Spam { autoexpunge = 10d } } }
I tried to use autoexpunge inside lmtp:
20-lmtp.conf:
protocol lmtp {
   mail_plugins = $mail_plugins sieve
   postmaster_address = ...
   info_log_path = /var/log/dovecot-lmtp.log
   sendmail_path = /opt/csw/sbin/sendmail
   namespace inbox { mailbox  Junk { autoexpunge = 10d } }   (line 23)
}
But I get an error during start of dovecot (same with service imap):
doveconf: Fatal: Error in configuration file 
/usr/dovecot/etc/dovecot/conf.d/20-lmtp.conf line 23: Garbage after '{'
/usr/dovecot/sbin/dovecot -n
# 2.2.21 (): /usr/dovecot-2.2.21/etc/dovecot/dovecot.conf
# Pigeonhole version 0.4.10
Is it only a problem of doveconf?
-- 
Regards --- Burckhard Schmidt
Abteilung Systemsoftware und Kommunikation
ZE Computer- und Medienservice der Humboldt-Universit?t zu Berlin
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Mail:          bschmidt at cms.hu-berlin.de