To verify a mail's signature you need the public key of the sender. You can normally get the public key (here called certificate) from the sender of the mail or from a public key server. But if you want to be sure, that you got no faked public key, you should verify it's fingerprint with the sender (e.g. by phone).
I am not sure if gpg4win's Outlook integration can handle signature verification at all. I for me self use gpg4o (gpg4o) which is much more reliable than the open source solutions and offers a very smooth integration into Outlook.
Edit: I just looked for the key and I found a public key for Daniel xxxx Hxxxx on the key server hkp://pool.sks-keyservers.net:11371