It is not recommended to dynamically prepare the URL at run time, especially based on ServletRequest. This is primarily because you have no idea of the URL that users would be using to access the application – the application server could be behind a web server, a firewall or a load balancer. To keep it short, one cannot predict network topologies.
Your current technique of fetching the URL from the property file is good enough to resolve the said issue. Maybe you should look at providing an administrative console to manage the URL appearing in mails, especially if there is an admin console in place, or if there are related options that should go into one.
Edit: My last point echoes what Tony has spoken of.