Symptoms

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

Jan 20 16:47:45 testserver postfix/trivial-rewrite[5708]: warning: hash:/var/spool/postfix/plesk/transport lookup error for "mail@example"
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 "test@gmail.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 <mail@example>: Temporary lookup failure; from=<mail@example> to=<test@gmail.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

Switch 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 :

# /usr/local/psa/admin/sbin/autoinstaller --select-release-current --install-component qmail
# /usr/local/psa/admin/sbin/autoinstaller --select-release-current --install-component postfix

Before installing back Postfix move its configuration files to another location, e.g.:

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

If it doesn't help restore mail configuration:

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