A lot of potential Rated 3 out of 5 stars

Looks great, but appears to have been abandoned. There have been no updates since 2007, and if you enable the DomainKeys function, it just gives an error saying, "DomainKeys checks are currently not available." It does have an option to point to a different "DK verification server," but gives no clues as to where one would be available. If anyone knows of any, please post it/them for the benefit of others who wish to use this feature while it is not available from its developer's site.

This review is for a previous version of the add-on (0.9.0.3). 

DomainKeys isn't supported

DomainKeys isn't supported anymore by the extension.

It wasn't abandoned so much as the extension worked and there wasn't anything to change. On the other hand, it probably is abandoned now since I don't have time to make it work with the upcoming Thunderbird 3.