if you were using the old version (1.x) of s2emails, please remove this table:
And install the plugin again. The reason is that we have completely revamped the tables in the DB.
In around 10 days we will upload a new version of the plugin, and you will have to update the DB manually because we will add a new option to send the emails when a user's role has been changed.
However, this time it will be easier and without the need of deleting tables - you will only add one new column.
This will be the last update with all this hassle. Now we have built in auto-updates that will do all the work for you in the future.