smtp mail question

Anyone see a problem if I login into the smtp server with Username different than the Return-Path? It seems to work OK; but, I know from experience using the mail servers that increasingly everything must be exactly right to prevent recipient mail servers from rejecting emails. So, I started using authenticated smtp exclusively. Reason I'm asking is that I'm developing an application that will have several pages and each one will have a different Return-Path and Reply-To. It will make things simpler if I can login to the smtp server with just one username/password. Al.........