I moved the site to a test subdomain on my own host.
After pressing send, the progress bar gets to around 73/74 of 100 before I get the error message
A communication error occurred: "" The Web Server may be down, too busy, or experiencing other problems preventing it from responding to requests. You may wish to try again at a later time.
For assistance, contact your network support team.
I tried changing the burst size and time, but it seems to make no difference.
I changed the wait time to 0.25 seconds ( can it go any lower ?) and it now processes all of the emails.
Is the problem due to server timeout ?