Failed to fetch webpage, temporary failure resolving domain

1.0 Error

While configuring a Linux server, the error Failed to fetch webpage, temporary failure resolving domain was observed.

$ sudo apt-get update
Err:1 http://mirrors.xxxxxx.com/ubuntu bionic InRelease
  Temporary failure resolving 'mirrors.xxxxxx.com'
Err:2 http://mirrors.xxxxxx.com/ubuntu bionic-updates InRelease
  Temporary failure resolving 'mirrors.xxxxxx.com'
Err:3 http://mirrors.xxxxxx.com/ubuntu bionic-backports InRelease
  Temporary failure resolving 'mirrors.xxxxxx.com'
Err:4 http://security.ubuntu.com/ubuntu bionic-security InRelease
  Temporary failure resolving 'security.ubuntu.com'
Reading package lists...
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Dovecot: Fatal: Error in configuration file /etc/dovecot/dovecot.conf: protocols: Unknown protocol: sieve

1.0 Error:

While configuring the Dovecot Mail Delivery Agent, the Dovecot server would not start and the following error messages were printed.

$ sudo systemctl start dovecot
Job for dovecot.service failed because the control process exited with error code. See "systemctl status dovecot.service" and "journalctl -xe" for details.
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SSL Certificate expired, site not opening

1.0 ERROR

A website stopped opening in browser, giving the error that the SSL certificate was either invalid or had expired.

2. SOLUTION

Checking the website with wget,

$ wget https://www.example.com
--2018-01-14 16:58:36--  https://www.example.com/
Resolving www.example.com (www.example.com)... 198.51.100.12, 2001:db8:1221:2341:f03c:f27b:12fa:2123
Connecting to www.example.com (www.example.com)|198.51.100.12|:443...
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