Yahoo emails sent from mac email program now have red untrusted signature

I sent an email on Wednesday that usually would have had a quick response. After a few days I checked my sent folder to make sure I had actually sent the email and not left it in a draft. When I opened the email I had, in fact sent, my name was red with a red question mark. When I clicked on my name it displayed untrusted signature and the signature displays not trusted. When I go into the Yahoo account via safari, I can an email and it does not display it in red with untrusted so I think it must have something to do with my Mac. I went into keychain but there are no expired certificates in either 'my certificate' or 'certificate' headings. My certificate reads 'self-signed root certificate' 'expires Thursday March 1st 2029' 'this certificate is marked as trusted for (my email is listed here)' How do I fix this. Thanks!