Spam Emails Appear To Be Sent By Me

A friend has reported that a dodgy looking email appears to have been sent by me. There is nothing in either of my 'Sent' folders for my 2 email addresses I use, and I assume this is some sort of malicious activity.
I've noticed there type of email from some of my contacts (they usually ask me to click a link to look at pics) but they appear obvious.
Do I need to just change passwords, or do I need to look for a new email client; I currently use Thunderbird.

 

It sounds like a simple spoof, you can send an email and put anybody's address as the sender / return address. If you don't see them in your outbox, you're likely safe.
Where you can look is in the message headers for IPs or servers used.

 

Given the recipient knows you, it is likely you or one of your mutual contacts have installed malware and it is sending e-mail to contacts harvested from the system it's running on. Such e-mails wouldn't appear in your sent items list as foogoo suggested.

I would run a full scan for malware as a precaution.

 

Your email has probably been hacked and the hacker has access to your contacts list enabling him to send out emails to your contacts, spoofing your address as sender. You should change your email password - that may help stop any more from being sent.