Google Likes changing their SSL Certs almost hourly. They normally switch between 2 or three, and every time the cert changes, Claws lets me know. Claws should only be notifying me if it's a previously unknown cert, not for switching between multiple known ones.
See "unsafe_ssl_certs" hidden preference at http://www.claws-mail.org/manual/claws-mail-manual.html#adv_hidden
There is also an option in your account preferences, on the SSL page: "Automatically accept valid SSL certificates". Use this.