I installed the latest version today and it scanned al my email. I had to restart and than I send a email to myself and got this from Thunderbird. Its Dutch but I try to translate a bit.
Have you signed up for your free certificate? Open “SecureEmail Configuration” from START>>PROGRAMS>>COMODO>>SECUREEMAIL.
Click on “Certificates” on the left and then Click on “Sign Up…” on the right side. This will take you to the Comodo Email Certificate Sign-up page where you simply input your name and email address. It will then send you an email telling you that you can collect your certificate.
Simply click on the link and download and install the certificate following the instructions.
You’ll probably have to reboot but once you do, it should be working fine.
Also check that "Digitally Sign Messages (by default) doesn’t have a tick next to it and that the Encryption on the same page is set as “never”. Comodo Anti-spam does the signing for you.