Thank you for your post.
If the queue stopped and says “Could not instantiate mail” function in most cases it means that your hosting has an hourly/daily/etc… sending limit on the local mail server. So the email server stops the sending, thus the error. And later, possibly the next day, they will automatically continue sending. We have seen this problem regularly, especially on shared hosting accounts that do have sending limits implemented to prevent spam.
Best is to ask the hosting provider what the email sending limit is and then you can setup your scheduling to accommodate that or you can ask them to increase it or alternatively, you can use a 3rd party SMTP relay service.
I can recommend some services like AuthSMTP, Amazon SES, SendGrid, Mandrill, etc. See some of them here on this documentation of ours:
* Amazon SES: https://docs.tribulant.com/wordpress-mailing-list-plugin/7131
* Mandrill: https://docs.tribulant.com/wordpress-mailing-list-plugin/9000
* SendGrid: https://docs.tribulant.com/wordpress-mailing-list-plugin/9007
Please note that with these SMTP services, the plugin makes a PHP socket connection to the SMTP host of the service. Some shared hosting providers block certain ports and some block outgoing PHP socket connections completely. You can just ask the hosting provider to open the port for you or enable fsockopen() accordingly. If you get a “Called Mail() without being connected”, that usually means the outgoing connection is blocked by the hosting. Again, the hosting provider can quickly open/allow it for you to resolve the problem.
Let me know if this is resolved once you’ve spoken to the hosting provider and if you need any further assistance on this.
