MailPress: Cannot send test emails, client cannot send emails

The client contacted me to say that after trying to send three newsletters on a new MailPress installation, all three appeared in different parts of the ‘All Emails’ display as ‘sent’, ‘pending’ and ‘unsent’, but all are definitely unsent. He then tried to send a test email to himself and received this error message:

Warning: fsockopen() [function.fsockopen
]: unable to connect to :0 (Failed to parse address “”) in /home/(domain info)/wp-content/plugins/mailpress/mp-includes/Swiftmailer/classes/Swift/Transport/StreamBuffer.php on line 233

Warning: Cannot modify header information – headers already sent by (output started at /home/(domain info)/wp-content/plugins/mailpress/mp-includes/Swiftmailer/classes/Swift/Transport/StreamBuffer.php:233) in /home/(domain info)/wp-includes/pluggable.php on line 934

I logged in and attempted to send a test email to myself and received the message “Test settings saved, Mail not send as required”.

I have experimented with different combinations of SSL, TLS, and ports, and none are working.

I tried both the Connection_php_mail and Connection_sendmail add-ons. Neither worked, and I got the same “Test settings saved, Mail not send as required” message with both.

There is nothing in the error log from the past week. There is nothing in my Cpanel error log at all. All we have to go on is that obnoxious “Test settings saved, Mail not send as required.”

https://www.ads-software.com/extend/plugins/mailpress/