dovecot - Sep 2016 - Alternate mail store of few users is disparaged unexpectedly

Hello,

We are using dovecot with mdbox and ALT store on S3(Simple Storage System).
We export the
mail_location=mdbox:/<user-mailstore>/mdbox:ALT=/altmailstore/<user-mailstore>/mdbox
of user in extra parameters.
Today, alternate mail store of few users is disparaged unexpectedly which
resulted in mail loss.
We are unable to trace out why and when the alternate store is disparaged.
Our hypothesis is, the folder is removed during doveadm altmove.
We want to prevent this in future.
It will be great if someone helps to identify the root cause.


Dovecot version: 2.2.19

# dovecot -n
# 2.2.19: /etc/dovecot/dovecot.conf
# OS: Linux 2.6.32-573.el6.x86_64 x86_64 CentOS Linux release 6.0 (Final)
auth_mechanisms = plain login
auth_verbose_passwords = plain
disable_plaintext_auth = no
mail_fsync = always
mail_gid = 65111
mail_location = maildir:~/Maildir
mail_nfs_index = yes
mail_nfs_storage = yes
mail_plugins = " quota"
mail_uid = 65111
mdbox_rotate_interval = 1 days
mdbox_rotate_size = 20 M
mmap_disable = yes
namespace inbox {
  inbox = yes
  location   mailbox Drafts {
    auto = create
    special_use = \Drafts
  }
  mailbox Junk {
    special_use = \Junk
  }
  mailbox Sent {
    special_use = \Sent
  }
  mailbox "Sent Items" {
    auto = create
  }
  mailbox "Sent Messages" {
    special_use = \Sent
  }
  mailbox Trash {
    auto = create
    special_use = \Trash
  }
  mailbox chat {
    auto = create
  }
  prefix = INBOX.
  separator = .
  type = private
}
plugin {
  mail_log_events = delete undelete expunge copy mailbox_delete
mailbox_rename
  mail_log_fields = uid box msgid size
}
ssl_cert = </conf/server/server.crt
ssl_key = </conf/server/server.key


-- 
Sincerely,
Prakash P. Autade.