Explanation, please, so I can avoid in the future?

Hi,
I have learned that we can't Blind Carbon Copy FTH addresses, but when I today and also somewhat recently Carbon Copied my area council members at their FTH addresses, I received the following message:WARNING: An email security check failed during email processing. Your email was still delivered.

(DKIM Signature Verification: fail (message has been altered))

Emails failing security checks may be blocked in the future.
If you have any questions about this, please visit support.toastmastersclubs.org .

Is there something I need to do here?

FTH will block any email that have a BCC

The issue is your sending email host is attaching a DKIM signature that fails verification.