Gegtor you'll need to edit the settings via a database admin tool (like phpMyAdmin). In the settings table, the relevant fields will be the ones beginning with mail_ . You can delete all lines starting with mail_ to restore default values, or edit them with the correct values.
Often this error is caused after setting an invalid mail_driver value. Setting it to smtp or blank should restore access to the forum. I believe in beta8 there will be a check so you can't enter an invalid value by mistake anymore.
