Cannot send message from Webmail: Temporary lookup failure

Symptoms

• Cannot send an email from Webmail: Temporary lookup failure

  SMTP Error (451): Failed to add recipient "john.doe@example.com" (4.3.0 <anna.roe@example.com>: Temporary lookup failure)

• SeLinux is enabled:

  # getenforce
  Enforcing

• The following error can be found in /var/log/maillog :

  # server postfix/trivial-rewrite[20662]: warning: hash:/var/spool/postfix/plesk/virtual_domains is unavailable. open database /var/spool/postfix/p
  lesk/virtual_domains.db: Permission denied
  server postfix/smtpd[5590]: NOQUEUE: reject: RCPT from unknown[192.0.2.2]: 451 4.3.0 john.doe1@example.com: Temporary lookup failure;
  from=john.doe1@example.com to=john.doe2@example.com proto=ESMTP helo=<[192.0.2.3]>

• Check of SeLinux context shows the following:

  # ls -Z /var/spool/postfix/plesk/virtual_domains.db
  -rw-r--r--. root root unconfined_u:object_r:user_tmp_t:s0 /var/spool/postfix/plesk/virtual_domains.db

Cause

Invalid SeLinux context.

Resolution

1. Log into the server via SSH.
2. Restore SELinux context for the Postfix files:

   # restorecon -R /var/spool/postfix

Note: In case the issue remains, check that SELinux is configured properly by following the resolution in the article below:
Unable to send and email: Unable to open file /var/lib/plesk/mail/srs/secret: Permission denied