This means your daily limit for your email account (gmail, zoho, outlook) has been exceeded.

Check with your email providers daily limits and increase it if possible.

Ideally if you get this message, you should give this email address a break for 1-2 days to prevent future blocks.

Steps:

• Go to your existing campaign, click the edit button
• Top right, select the settings button (the cog wheel)
• In the SMTP tab, untick the email address causing issues
• Click save and voila