I would be worried to. Someone has used your email address or spoof a spam (or worse) he could be sending those mails from your pc or your mailserver. Or he just pretended beeing you (spoofed). Check in message source if your mailserver are the sender of original message.

Run several anti-malware programs (there are many free ones, but not all are trustworthy)

Malwarebyte >> http://www.malwarebytes.org/mbam.php Microsoft Safety Scanner >> http://www.microsoft.com/security/scanner/en-us/default.aspx Windows Defender >> http://windows.microsoft.com/en-us/windows/using-defender Spybot >> http://www.safer-networking.org/en/index.html Kasperky >> http://www.kaspersky.com/security-scan Rootkit scanner TDSS-Killer >> http://support.kaspersky.com/5350

Go into your mail account on server, using webmail and change your password. Remember to delete saved password for this account (both IMAP/POP and SMPT) in Tbird (options /security /passwords /saved passwords )

You can enter it next time you log on

Probably not too much to worry about. Sounds like a spammer used your email as From: address to send spam messages, and now the ones which couldn't be delivered are coming back to you. It's called backscatter. https://en.wikipedia.org/wiki/Backscatter_(email)

It will stop at some point. Until then you can create a filter to delete them right away. http://kb.mozillazine.org/Filters_(Thunderbird)

Two great answers thanks. I changed my password before I got on to Virgin Media. A technician there said that what I had been told was correct in each case. He went on to say that it would probably stop of its own accord but if it didn't I was to get back to him. So OK now.