The default value is: ^[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,4}$
Change it to: ^[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,}$
The difference is that the last part of the e-mail address has no limitations on the number of characters.
var builder = new ContainerBuilder();
// Register your types
// ...
var container = builder.Build();
// Set the resolver to Autofac
DependencyResolver.SetResolver(new AutofacDependencyResolver(container));
// Sitecore WFFM
builder.RegisterType<Sitecore.Forms.Mvc.Controllers.ModelBinders.FieldBinders.DefaultFieldValueBinder>().AsSelf();
builder.RegisterType<Sitecore.Forms.Mvc.Controllers.ModelBinders.FieldBinders.CaptchaFieldBinder>().AsSelf();
builder.RegisterType<Sitecore.Forms.Mvc.Controllers.ModelBinders.FieldBinders.CheckboxFieldValueBinder>().AsSelf();
builder.RegisterType<Sitecore.Forms.Mvc.Controllers.ModelBinders.FieldBinders.DatePickerFieldValueBinder>().AsSelf();
builder.RegisterType<Sitecore.Forms.Mvc.Controllers.ModelBinders.FieldBinders.ListFieldValueBinder>().AsSelf();