LDN - Luminis Developer Network

Currently we are running the Luminis 3.3.X and we are looking into upgrading to Luminis IV. However, we are also looking into moving to an external emailing system. Currently, we don't know of any but exchange server that people have used. I wanted what others are using for external emailing systems, how is working for them and if they know of any other systems. We are in the process of finding the pros and cons of external emailing and if we should implement it.