Sending messages to a large number of recipients usually takes rather long. In this case the process may be terminated due to technical limitations on your web server. To keep the sending process executing until the campaign is completed, it is often sufficient not to close your browser tab with Mail app open. If you do this, the script executed in your browser will regularly resume the sending process should it be interrupted.
There is also a more reliable method to continue sending messages regardless of the server limitations which requires setting up a special server scheduler (cron). With Mailer's cron jobs set up, the scheduler will execute them with specified frequency until a message has been sent to all recipients. In this case you do not need to keep your browser tab with Mailer app open.
In Webasyst Cloud you do not need to set up anything additionally. If you have installed Webasyst on your web server, then it is required to add cron jobs for sending long email campaigns.