Summary
When sending several 1:1 emails in a row, a CAPTCHA appears with an alert that says, "Please complete the CAPTCHA form to send your next email."
Resolution
Wait 30 seconds before sending the following 1:1 email. Or, solve the CAPTCHA to send the 1:1 email.
Cause
A maximum of two emails can be sent within 30 seconds (from the same IP address).
If you send a third one within 30 seconds, the system will start to display a CAPTCHA that you'll need to solve before being able to send the email.