Unable to send an email via WordPress mail plugin: stream_socket_enable_crypto(): Peer certificate did not match expected

Symptoms

• WP-Mail-SMTP plugin is installed on WordPress site.
• It's not possible to send a test email WP Admin > Settings > Email > Send test:

  CONFIG_TEXT: Warning: stream_socket_enable_crypto(): Peer certificate CN=`server.hostname.com' did not match expected CN=`localhost' in C:\Inetpub\vhosts\example.com\httpdocs\wp-includes\class-smtp.php on line 369
  ...
  SMTP ERROR: Failed to connect to server: (0)SMTP connect() failed

• Plugin is configured to use port 587 or 465: WP Admin > Settings > Email > SMTP Port

Cause

Incorrect SMTP Host is specified in plugin.

Resolution

Use server hostname(server.hostname.com) in WP Admin > Settings > Email > SMTP Host instead of localhost.

Hostname can be found at Plesk > Home page or in  MailEnable > Services > localhost > Services  and Connectors > SMTP Properties > General > Local domain name

Note: Mail server must be secured by SSL certificate .

Alternative solution by disabling STARTTLS is described at WordPress with SMTP extension cannot send mail.