Unable to connect SMTP server:smtpout.mail.zoho.com Reason:Authentication failed:554 5.1.8 Sender Address Blocked

I am trying to send out an email with all of our customers in the BCC area and even though I have broken up the amount of emails I include in this area I keep getting this error message

Please let me know how I can send out emails to my entire customer base without getting this error message.