smtp/config.go
func (se *SMTPEnvironment) UnmarshalYAML(unmarshal func(interface{}) error) error {
	var aux struct {
		Env map[string]struct {
			Config Config `yaml:"smtp"`
		}
	}
	if err := unmarshal(&aux.Env); err != nil {
		return err
	}
	se.Env = make(map[string]Config)
	for env, conf := range aux.Env {
		se.Env[env] = conf.Config
	}
	return nil
}
func (se *SMTPEnvironment) UnmarshalYAML(unmarshal func(interface{}) error) error {
	var aux struct {
		Env map[string]struct {
			Config Config `yaml:"smtp"`
		}
	}
	if err := unmarshal(&aux.Env); err != nil {
		return err
	}
	se.Env = make(map[string]Config)
	for env, conf := range aux.Env {
		se.Env[env] = conf.Config
	}
	return nil
}
//UnmarshalYAML implements Unmarshaler. Avoid use of env in the YAML file.