If its all on your last email, then you probably answered to all or all in the list instead of a single person

I just looked again and the codes in the error message are '2,3,2,3' and '2,4,2,3'

I don't know if that sheds any light on the subject?

Look at the mail you sent and the mail you replied to, look at the code (shortcut Ctrl+U) if they have multiple recivers (TO:)

Thanks, I will try & get back to you later.

Unfortunately there doesn't seem to be a list of multiple recipients - just my email address and the person I sent to. (Then much code type gobbledigook).

Could my inbox / sent items actually be full? Or can you have hundreds of messages stored in Thunderbird?

It's most perplexing!

If those addresses in the returned email isn't any you know, and the content isn't what you said then most likely someone has used your email-address as sender ("FROM:") in a spam-mail. If it is your content that got to all those unknown people then I would suggest you scan your pc for virus, and use several different

I'll start an AVG scan now. Fingers crossed!

Change the password on you e-mail account NOW !

Sometimes a problem with Firefox may be a result of malware installed on your computer, that you may not be aware of.

http://www.clamxav.com/ free malware scanner for Mac OS X https://discussions.apple.com/docs/DOC-3291

Windows: You can try these free programs to scan for malware, which work with your existing antivirus software:

• Microsoft Safety Scanner
• MalwareBytes' Anti-Malware
• Anti-Rootkit Utility - TDSSKiller
• AdwCleaner (for more info, see this alternate AdwCleaner download page)
• Hitman Pro
• ESET Online Scanner

Microsoft Security Essentials is a good permanent antivirus for Windows 7/Vista/XP if you don't already have one.

Further information can be found in the Troubleshoot Firefox issues caused by malware article.

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