Signed error

I installed a Comodo email certificate on Thunderbird. I sent an email to another email of my own and I have the following advise when using Thunderbird.
Digital signature is not valid
The certificate used to sign the message was issued by a certificate authority that you do not trust fort issuing this kind of certificate
How can I correct this in my computer? or What I am doing wrong?

I open it with Outlook and it says the digital signature is valid and trusted

I got it. you have to go with mmc to the certificates folder then Intermediate Authorities and export the certificate as .cer to a folder and then import it on thunderbird in Authorities.