There is no maximum (and even if there is, it is far far more than nine), so your problem is the email address. Maybe if you post the complete message, we can assist in locating the problem.

Thanks for your reply. The email address is valid. If the so-called invalid address is then removed and the email is resent, then it will just pick another valid email address to reject, and so on until it winds up rejecting them all. I have attached an image of the error report that pops up. Is there any setting that can change this?

Have you checked whether your ISP allows more recipients? Some have limits for a single message.

Thanks for your suggestion. I haven't changed my ISP, so can't imagine it has suddenly changed its limit. For a long time, I have included over 30 BCC recipients when sending my email and have not had a problem. Now, all of a sudden, when I try to send the same amount of BCC recipients, it won't send anything over 9 and rejects valid addresses for any over that number; until each of them is eliminated from the list, thus reducing the number down to 9, the email will not be sent. I want to know if there is any way I can adjust this number - I can't find anywhere in my computer to make such a change. Thanks.

Thanks for your suggestion. I haven't changed my ISP, so can't imagine it has suddenly changed its limit.

Unfortunately mail providers change requirements all the time. Thinking they will not is really self defeating.

For a long time, I have included over 30 BCC recipients when sending my email and have not had a problem. Now, all of a sudden, when I try to send the same amount of BCC recipients, it won't send anything over 9 and rejects valid addresses for any over that number; until each of them is eliminated from the list, thus reducing the number down to 9, the email will not be sent.

I have never heard of a provider limiting the number of BCC recipients specifically. Generally it is a total recipients limit for To, CC and BCC inclusive.

I want to know if there is any way I can adjust this number - I can't find anywhere in my computer to make such a change. Thanks.

Perhaps if you actually advise which mail provider the discussion was about there might be a chance of identifying exactly what the issue was. But as even your email address is hidden from us, we have no chance of identifying even which continent you are from. Statistically probably North America, but the second biggest user base is Europe.

As you were advised originally Thunderbird has not limits. There might be some technical limit that the total of recipients is limited to the amount of available memory or something silly like the absolute length of the string allocated to hold the address data, but there is not practical limit from the perspective of the user.

Your error message mentions a rogers email address, I know it is not yours, but Rogers email is provided by Yahoo, and Yahoo change what is and is not acceptable all the time. Just ask the people trying to use a mail client, any mail client with ATT or AOL accounts. Also provided by Yahoo

It is most unlikely your issue has anything to do with Thunderbird, despite your reluctance to accept that. So looking for setting on your device to change the refusal of your providers server to accept mail with more than XX BCC addresses is not going to be the answer. Understanding the rule is probably the answer.

Regarding the email address that are rejected - do they share the same domain or share same domain as those already in list ?

I ask because Server Spam filters are getting hot about sending Bcc to several email addresses that share same domain name. This is because it is typical for real spammers to use that method.