There can be several reasons why LUCY might stop sending mails after a campaign has been started:

• Too high memory usage (fixed in 3.0)
• The user can save advanced system settings or change campaign configuration during a campaign. That will trigger the background worker queue to restart and may result in stopping the email queue (fixed in 3.0)
• There might be some email formatting issues (fixed in 3.0 as well) that do not allow to process a mail

In such a case you can try to STOP the campaign and then START it again. Do not use the restart function! When you simply START the campaign again, LUCY will try to continue sending mails right where it stopped. There won't be any mails send to recipients again who already received a mail.