I was getting the same error (when sending to more than 247 subscribers in one pass) after I moved to a new hosting provider on a server that was 5 to 10 times faster than my old hosting provider. My new provider has a limit on how many resources I can use simultaneously. It turns out I was exceeding one of those parameters (I believe it was number of processes or IOPS, I can’t recall which one) so my ability to send email from PHP was suspended for a few seconds. That is when I received the errors. I fixed it by using define(‘MAILQUEUE_THROTTLE’, 1.00); in config.php. All my lists are less than 500 subscribers, so a one second delay between messages is not a problem for me.
There are, of course, many other items that can cause the “Could not instantiate mail function.” error. I’m just letting you know what I did to solve my problem.
Without reading too much of this already big thread. You need to make your session timeout longer and the intervals between the mails should be small so you don’t get out of session. For example send every 10 minutes 3 messages, so you don’t get out of session.
Second. Please shut up this provider, because this embarrassing. 75 mails per hour in 2017