There are four main situations that would place a campaign on hold: 

Situation 1: You're a New MyBizMailer Client

Situation 2: Hard-Bounce Rates of More Than 10% On Campaigns

Situation 3: A Complaint Rate of 0.05% or Higher On Campaigns 

Situation 4: Insufficient Credits 

For a detailed in depth look at these four situations and what you can do to get your campaign sent please read: How Does the Hold and Approval Process Work?