Hello List

This is probably a dumb question and not really an issue for Notmuch.
But it is when using notmuch (through emacs) I get this Gnome pop-up.
See attached image. Some senders are attaching some sort of signature
that I get to trust or cancel. What does people do in this case, I tend
to cancel it. How should I relate to the question. How do I know if I
could ultimately trust something as asked.

The pop-up in this case says

----------------------------------8<----------------------------------
Message

Do you ultimately trust
"CN=GlobalSign
OU=GlobalSign Root CA-R3
O=GlobalSign"
to correctly certify user certificates?
---------------------------------->8----------------------------------

Is there some source to go to for verifying the cert (or issuer or
what it is).

PS: When a signature like this are sent, there will be a delay before
the pop-up.

Best regards
--
Tomas