Okay, I've spent the morning trying to figure out how to downgrade to version 7.3.3.3. The cleanest way to do this is to delete the plugin from the Plugins page of the dashboard and then to install the older version. The catch here is that it deletes the forms data from the database. This forms data is stored in wp_posts (WHERE post_type LIKE '%wpcf7%') and wp_postmeta (look for meta_key '_mail', '_mail_2' and '_form' - I think that's all but I might be wrong).
I've sent a test mail and am waiting now for it to come through. It's probably going to take a while though.