The information is set on the form on the settings tab and is later used in the actor that sends emails called SendEmailAfterSubmissionActor. I can't see a way to get to that information using the above events so I'll do it another way. 

So to set a default value for the from-value you could either hook into when form is saved (the emailsettings are stored on the form content type) or add some logic to when the actor is run. I chose the latter of the two.

The actor is executed by a class called ActorsExecutingService so I'll just modify slightly what this class is doing when it's loading the actors that should be run. 

public class ImprovedActorsExecutingService : ActorsExecutingService
    {
        public const string DefaultEmail = "info@mogul.com";
        public override IEnumerable<IPostSubmissionActor> GetFormSubmissionActors(Submission submission, FormContainerBlock formContainer, FormIdentity formIden,
            HttpRequestBase request, Guid permanentFormSubmissionId, bool isFormFinalizedSubmission)
        {
            var actors = base.GetFormSubmissionActors(submission, formContainer, formIden, request, permanentFormSubmissionId, isFormFinalizedSubmission);
            var emailActor = actors.OfType<SendEmailAfterSubmissionActor>().FirstOrDefault();
           
                var emailModels = emailActor.Model as IEnumerable<EmailTemplateActorModel>;
                foreach (var emailModel in emailModels)
                {
                    if (string.IsNullOrWhiteSpace(emailModel.FromEmail))
                    {
                        emailModel.FromEmail = DefaultEmail;
                    }
                }
            return actors;
        }
    }

Of course you also need to tell EPiServer to use your new improved version :)

You can do this when configuring the ioc container

   private static void ConfigureContainer(ConfigurationExpression container)
        {
            container.For<ActorsExecutingService>().Use<ImprovedActorsExecutingService>();
...
}

There is probably an easier way to do this...or there should be at least but it's a fun way of doing it :)

Thank you for your help Daniel! :) It worked out with the solution you provided.

Sweet! I didn't try setting the from address in smtp tag in web.config I just realized. That would of course be easiest if it worked. Probably should be implemented that way if not... :)
But it was a fun coding exercise anyway...