Every SMTP server or mail server API enforces its own limit, and the Gmail API is no different. The Gmail API limit is hard-fixed at 100 recipients per email. Because of this, your emails may fail the initial sending when sending an email which puts the recipients over this or other Gmail limits. To, CC and BCC recipients all count towards the limit. You can bypass this issue by cloning your email with the help of the clone tool to split the recipients into identical emails.