Sending limits are imposed by your mail provider, not Thunderbird. What the limits are is only knowable with searching 5the term and conditions of the relevant email provider you send through.

Essentially your mail provider is reporting that you have already sent as many mail as you are allowed to. This can be many hundreds of mails in a day or hour, or it might be as few a 1 if you are not authenticating with the server. That is you are not authenticated so your limit is Zero.

Given Google are a huge proportion of the market and you say nothing about who you use. Here are the google limits in case they are the relevant ones https://support.google.com/mail/answer/22839?hl=en

Thanks for the idea. But I already talked to my email provider who is local, not Google. They could see no problem and through their web mail portal, I can send messages. It is only with Thunderbird that I get this problem.

No, it is through any email client that is improperly configured. As @matt indicated, you may have problems in your configuration. Thunderbird does not create messages, it only presents messages it receives from server. You share nothing of your setup, so no advice can be offered.

I am using Peak Internet in Corvallis, Oregon. The thing is, up until Sunday sometime, I had no problem sending emails. This problem came up just out of the blue. Nothing had changed in my computer that I know of. Does "F13" originate somewhere in Thunderbird? I am using the latest version of Windows Home edition.

Thunderbird is an email client. It does not care how many messages you send or their content. The messages it presents to you are those that are received back from the server. It is also not uncommon for tech support persons for an email provider to not be aware of the various error messages that it sends. Possibly, if you post a screenshot of your SMTP server pane, some suggestions may surface.