You're wanting to send to too many recipients at once. Contact your email provider for more information.

Thank you David On one computer I can still send a bulk email, but on the other (using the same software) it comes up with an error. One would think that there is a "glass half empty" in one that is local to the computer. Possible?

@vicjackson Thunderbird is a client software, it works with a server, a mail server on the Internet, that is managed by a mail provider. If you install Thunderbird on 2 computer but on one computer it's configured to connect to a mail server managed by provider1, it may work differently with the Thunderbird installed on the other computer configured to connect to a mail server managed by provider2. The denial comes from the provider, not from the computer where the client software (Thunderbird) is running.