Reading the Authentication-Results header should be the normal behavior, since DKIM was designed to be verified by the receiving server. However, each server should have its own settings and policies (for example, some servers don't bother writing results for failed signatures, since they should be treated as non-existing.)
This add-on highlights those results by reporting them on the message window, along with From:, Subject:, and To:. Kudos for that!
Doing the crypto is good for servers that don't do it themselves, and for manual action. Such late verification may fail for old messages.
This review is for a previous version of the add-on (1.3.6).