Warning: stream_socket_enable_crypto()

Getting the following error on several of my sites. Seems to have started after the most recent WordPress update.

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 /public_html/wp-content/plugins/cimy-swift-smtp/Swift/lib/classes/Swift/Transport/StreamBuffer.php on line 95

https://wordpress.org/plugins/cimy-swift-smtp/