Error: "Reason;5.5.0 Recipient already specified" - I can't send email!

Error: "Reason;5.5.0 Recipient already specified" - I can't send email!

I just send a message through the 'Help Center'/'Write to us' option in ZOHO-Mail, but I will post the same question again here, because I am trying to send a rather important, time-sensitive business email.

I am trying to send an email to all my customers with an important announcement, so there are many recipients in the bcc area. In the 'To:' field I have my own e-ddress (that is how I have been sending group-mails before, while preserving the privacy of my clients).

I am getting an error message: "Unable to send message; Reason: 5.5.0 Recipient already specified" - ( see attachment). I understand the words, but I don't know what it means. How can I send this time sensitive message?