SMTP Certificate verification failure

Error message/code:

Warning: stream_socket_enable_crypto(): SSL operation failed with code 1.
OpenSSL Error messages: error:14090086:SSL routines:SSL3_GET_SERVER_CERTIFICATE:certificate verify failed

Description:

PHP versions since 5.6 verify certificates on SSL connections.
If there's a problem relating to the certificate, you will get an error like this.

Solution:

See this article