With our latest version 4.5.7 some users have experienced an SMTP connect() failed error message with our WordPress Newsletter plugin when trying to send newsletter emails using SMTP relay.
phpMailer was upgraded to version 5.2.10 in this version of our Newsletter plugin and it introduced an automatic TLS protocol feature to check if the mail server supports it and then automatically encrypt the data over TLS. Read more about that here: https://github.com/PHPMailer/PHPMailer/wiki/Troubleshooting#opportunistic-tls
Unfortunately this may cause the SMTP connect() failed error depending on what your server setup is.
If you experience this, we will shortly release a new version which turns the opportunistic TLS feature off by default. In the meanwhile, you can submit a ticket to us for a new version/build or you can open wp-mailinglist/vendors/class.phpmailer.php and set the $SMTPAutoTLS variable to false and save the file.
Thank you for your understanding!