Cloud platform has a limit of 200 emails per hour. If you reach that limit no further email will be sent out until an hour pass. You might get authentication errors once you exceed the limit. Also bare in mind that mailing lists are not allowed. There is a limit on concurrent connections and you account will be suspended automatically if you try to send large mailing. For mailing list please see our email marketing plans at https://guru-host.co.uk/en/Support_Services/Email_Marketing.html