code: func (c *Config) UPN(username string) (string, error) {
  	if _, err := mail.ParseAddress(username); err == nil {
  		return username, nil
  	}
  
  	domain, err := c.Domain()
  	if err != nil {
  		return "", err
  	}
  
  	return fmt.Sprintf("%s@%s", username, domain), nil
  }
docs: //UPN returns the userPrincipalName for the given username or an error if misconfigured.