Невозможно заблокировать сообщение об ошибке на SMTP-сервере

У меня появилось следующее сообщение об ошибке: 3 непрерывных для одного и того же почтового ящика

> (delivery failed to mailbox
> /var/spool/postfix/vhosts/<user>/mail/inbox: 
> 
> unable to lock for exclusive access: Resource temporarily unavailable

Я не понял, что происходит ... Вы можете мне объяснить?

Что я могу сделать, чтобы снова избежать появления такого сообщения об ошибке?

Кроме того, у меня есть следующие настройки:

# used only for creating mbox dotlock files when creation fails for INBOX.
# Rely on O_EXCL to work when creating dotlock files. The default is to use
# hard linking. O_EXCL makes the dotlocking faster, but it doesn't always
#dotlock_use_excl = no
# Locking method for index files. Alternatives are fcntl, flock and dotlock.
# Dotlocking uses some tricks which may create more disk I/O than other locking
# methods. NFS users: flock doesn't work, remember to change mmap_disable.
#lock_method = fcntl
# Which locking methods to use for locking mbox. There are four available:
#  dotlock: Create <mailbox>.lock file. This is the oldest and most NFS-safe
#  fcntl  : Use this if possible. Works with NFS too if lockd is used.
#  flock  : May not exist in all systems. Doesn't work with NFS.
#  lockf  : May not exist in all systems. Doesn't work with NFS.
# You can use multiple locking methods; if you do the order they're declared
# in is important to avoid deadlocks if other MTAs/MUAs are using multiple
# locking methods as well. Some operating systems don't allow using some of
#mbox_read_locks = fcntl
#mbox_write_locks = fcntl
# Maximum time in seconds to wait for lock (all of them) before aborting.
#mbox_lock_timeout = 300
# If dotlock exists but the mailbox isn't modified in any way, override the
# lock file after this many seconds.
#mbox_dotlock_change_timeout = 120
  # Keep the mailbox locked for the entire POP3 session.
  #pop3_lock_session = no
# blocking passdb and userdb queries (eg. MySQL and PAM). They're
    # [blocking=yes] [session=yes] [setcred=yes]
    # Setting blocking=yes uses the alternative way: dovecot-auth worker
    # [blocking=yes] - See userdb passwd for explanation
    # [blocking=yes] - See userdb passwd for explanation
    # [blocking=yes] - By default the lookups are done in the main dovecot-auth
    # proceses. Useful with remote NSS lookups that may block.
# used either directly or though a dictionary server. The following dict block