When a campaign is sent we want to make sure that the bounce rate remains under 5% and the complaint rate under 0.25%. The bounce rate is calculated by dividing the permanent bounces by the mails currently sent for this campaign. We don't count soft bounces in this rate (like mailbox full, or an out of office reply). For the complaint rate we divide the complaints by the mails currently sent for this campaign. A complaint occurs when a recipient hits the mark as spam button in the email client or moves the message to the spam folder.
The reason we do this is to protect your sender reputation. Email Service Providers calculate a reputation for your domain and ip address. If you send a lot of mail that bounces or causes complaints, this will have a negative effect on your sender reputation. If your reputation is low, your mail will more likely end up in the spam folder or even worse, not accepted by the recipient's mail server at all. That's why we stop your campaign when you accidently send a campaign to a lot of non existing email addresses. Also, we don't want our system to be used to send spam, because we all receive enough spam anyway. :)
Unfortunately, there is no way to restart a campaign once it is stopped due to too many bounces or complaints. If you believe this is a mistake and all bouncing addresses accidently ended up in the first mails sent, you can take the following steps to resend your campaign.