The server stopped repsonding.
It may be that the host has limits on the number of emails that can be sent in a specific time period, and once that limit is reached, it stops responding.
in the mail settings, there are settings to throttle the number of email sent per minute, and the wait time between emails, you will need to experiment with these settings to find a send rate your host will allow.
if you stop the mailing, then start it again, it will only send to the recipients it hasn't already sent to.
