What is the exact error message? What mail provider?

Both are important as most sending issues of this kind are caused by two or tree issues.

1. You are sending more messages than your provider allows in one go or one hour or one day.

2. Your provider does not like the mail you are sending with so many BCC recipients and blocks it as spam.

3. Your recipients mail servers or some of them identify the message with BCC as SPAM are refuse delivery.

With the exact error message and the provider we might be able to work out which applies, or if it is one of the more infrequent causes.

The exact error message is:

HE1EUR01FT056.mail.protection.outlook.com rejected your message to the following email addresses:

"ronbales1@gmail.com; robtapril"@hotmail.com A communication failure occurred during the delivery of this message. Please try to resend the message later. If the problem continues, contact your email admin.

HE1EUR01FT056.mail.protection.outlook.com gave this error: Requested action not taken: mailbox unavailable.

This was a test message. Only 2 recipients. Both sent as BCC recipients. I get similar error messages when I send to a primary recipient and 1 or 2 BCC's.

Did you put in the semicolon? Whatever has returned this message seems to be mis-parsing the addresses. See where the quotes are placed?

Thunderbird works best with one address per line. If you really must stack up multiple addresses on one line, use commas to separate them.

Thanks. Problem solved. I separated the emails by comma instead of semicolon. Works fine now.