Hi Mary Feild,
Have you tried a triggered Smart campaigns that uses the "opens sales email" and "clicks link in sales email" triggers, and a send alert or a add task flow step ?$
Another way to look at this is to look into the activity log of the lead that has been sent a sales email though the plugin. If something shows in the activity log, it's likely you can set t trigger on it.
BtW, the same question might be asked for Outlook plugin, is someone can give it a try ?