The error message seems pretty clear to me. If there is something you didn't understand you should contact your email provider. The message is from their server.

server people say it is Mozilla that is causing the problem as no matter what server I send to it comes with that error.

Unfortunately some email error message numbers can be a little flaky, i.e. they do not always correspond to the real error and hence can be misleading, either because they are combined in one message or not. If you run an internet search for them, you’ll probably find different explanations. Also see [http://forums.mozillazine.org/viewtopic.php?f=39&t=2812787 this mozillaZine topic] for more info. As you can see there and may already know, Frontier has a [http://postmaster.frontier.com/ page] referring to [http://postmaster.frontier.com/error.html error messages] including this one, indicating that this "exceeded hourly recipients threshold" could mean your computer is infected and therefor sending more messages (i.e. spam) than you may be aware of, so I would try to rule that out first. * Try scanning your system for malware and viruses (see below.) The issue could of course be temporary (since a counter/time is involved), but that doesn’t mean the root of it is gone as soon as you can send again. * You ''could'' switch to another SMTP server (either temporarily or not), but that would be a workaround. * If you are able to monitor outgoing traffic from your computer, or temporarily change e.g. Thunderbird’s port for outgoing messages in order to disable it (though any ''real'' sent messages should appear in the Sent folder), or block the outgoing SMTP port for your entire system, you should also be able to find out what is happening, though a malware scan is definitely required an often easier to accomplish for most users. You can try these free programs to scan for malware, which work with your existing antivirus software: * [http://www.microsoft.com/security/scanner/default.aspx Microsoft Safety Scanner] * [http://www.malwarebytes.org/products/malwarebytes_free/ MalwareBytes' Anti-Malware] * [http://support.kaspersky.com/viruses/disinfection/5350 Anti-Rootkit Utility - TDSSKiller] * [http://general-changelog-team.fr/en/downloads/viewdownload/20-outils-de-xplode/2-adwcleaner AdwCleaner] (for more info, see this [http://www.bleepingcomputer.com/download/adwcleaner/ alternate AdwCleaner download page]) * [http://www.surfright.nl/en/hitmanpro/ Hitman Pro] * [http://www.eset.com/us/online-scanner/ ESET Online Scanner] [https://support.microsoft.com/help/14210/security-essentials-download Microsoft Security Essentials] is a good permanent antivirus for Windows 7/Vista if you don't already have one. Windows 8/10 have antivirus protection [https://www.microsoft.com/windows/windows-defender built-in]. However and as discussed in the mozillaZine topic referred to above, the issue can still be caused by your ISP, i.e. on the server end. Unfortunately a lot of ISP’s can be in denial at first simply because one can be the first reporting an issue, or they may be less familiar with software like Thunderbird or even call it "unsupported". Did this help? Please report back.