The WFFM module was been significantly refactored in Sitecore 8.1+ and now allows for configuration driven dependency injection. The old Send Email Save Action has now been changed to
Sitecore.WFFM.Actions.SaveActions.SendMessage in Sitecore.WFFM.Actions.dll.
Factory Object Name field in the
Send Email Message item is now set to the following:
The setting points to a configuration node, which is defined in Sitecore.WFFM.Dependencies.config.
<sendMessage type="Sitecore.WFFM.Actions.SaveActions.SendMessage, Sitecore.WFFM.Actions">
<param name="settings" ref="/sitecore/wffm/settings" />
<param name="mailSender" ref="/sitecore/wffm/mailSender" />
The Save Action is carried out by the class specified in the type attribute.
Note that the constructor and method signatures have also changed since earlier versions so you will need to refactor your code but the change is not significant.