The most common cause for this is when users exceed the daily limit of their Email API Plan. One example is when you're on Sendgrid's free plan. Sendgrid's free plan allows you to send only up to a hundred emails per day. If you are on Sendgrid's free plan and you have (a) A campaign that sends more than a hundred emails per day, or (b) multiple campaigns that are sending more than a hundred emails per day combined, then your campaigns might pause once you've exceeded the 100 emails per day limit. You'll also see an error in your campaigns list that indicates that. But normally, it will resume the next day. If this isn't the case, please kindly reach out to support.