As an email service provider (ESP) it's vital for us to maintain the highest level of deliverability for our customers. If we don't, email clients such as Gmail and Outlook will potentially throttle incoming email where Drip was the sending source. To prevent that from happening we've implemented a broadcast send limit for all accounts currently in the trial phase.
During that time, broadcast sends are limited to two per day for up to a maximum of 5000 subscribers. Doing so allows us to vet that an account won't pose the risk of racking up unacceptable bounce or spam rates.
To lift the sending restriction, go to Settings > Billing located in the upper right. Click the Start Paying Now button to end your trial early and start your paid plan.