This is a legitimate link-shortening and redirection service owned by Microsoft. Like any link-shortening service, you should use a link-expanding service, such as wheregoes.com, to uncover the landing page *before* you click a short-link. Unlike many link-shortening services, aka.ms is unlikely to send you to bad landing pages because it is monitored.
Microsoft uses this service to obfuscate links, breaking Twitter's preview function, and I have had MS employees admit they use this to track visitors.