Unable to send emails: Temporary lookup failure

Symptoms

• Unable to send emails, in /var/log/maillog following information can be found:
  

  CONFIG_TEXT: Jan 20 16:47:45 testserver postfix/trivial-rewrite[5708]: warning: hash:/var/spool/postfix/plesk/transport lookup error for "mailbox@example.com"
  Jan 20 16:47:45 testserver postfix/trivial-rewrite[5708]: warning: transport_maps lookup failure
  Jan 20 16:47:45 testserver postfix/trivial-rewrite[5708]: warning: hash:/var/spool/postfix/plesk/transport is unavailable. open database /var/spool/postfix/plesk/transport.db: Invalid argument
  Jan 20 16:47:45 testserver postfix/trivial-rewrite[5708]: warning: hash:/var/spool/postfix/plesk/transport lookup error for "jdoe@example2.com"
  Jan 20 16:47:45 testserver postfix/trivial-rewrite[5708]: warning: transport_maps lookup failure
  Jan 20 16:47:45 testserver postfix/smtpd[5877]: NOQUEUE: reject: RCPT from localhost[127.0.0.1]: 451 4.3.0 <mailbox@example.com>: Temporary lookup failure; from=<mailbox@example.com> to=<jdoe@example2.com> proto=ESMTP helo=<testserver.test>
  Jan 20 16:47:45 testserver /usr/lib64/plesk-9.0/psa-pc-remote[22311]: Message aborted.

• Following command that restore mail configuration hangs:
  

  # /usr/local/psa/admin/sbin/mchk --with-spam

• Postfix package is corrupted:
  

  # rpm -V postfix-2.10.1-6.el7.x86_64
  .......T. c /etc/postfix/access
  .......T. c /etc/postfix/canonical
  .......T. c /etc/postfix/generic
  .......T. c /etc/postfix/header_checks
  S.5....T. c /etc/postfix/main.cf
  S.5....T. c /etc/postfix/master.cf
  .......T. c /etc/postfix/relocated
  ................

   

Cause

Corrupted Postfix package.

Resolution

1. Connect to the server via SSH;
2. Backup Postfix configuration files:
   

   # mv /etc/postfix/main.cf /root/main.cf.saved
   # mv /etc/postfix/master.cf /root/master.cf.saved

3. Switch Mail Transfer Agent(MTA) from Postfix to Qmail and back to Postfix that is described in [How to] How to define which MTA is used in Plesk and how to switch from QMail to Postfix and back
4. If it doesn't help restore mail configuration:
   

   # /usr/local/psa/admin/sbin/mchk --with-spam