It is a reallly nice discussion. About a solution a bit sensitive to trials and tribulations outside (Google). I had a similar situation where I didn't want all my internal surveillance system EMail notification traffic (as well as other admin notifications from other "equipment") going out and back in to my local net. My ISP was tolerant, and so was GMail, but...why DO that if you have an alternative? Running a system...as Zargon and others do...I run Ubuntu and another 3 systems running capable OS's. I had already set up an internal threesome of DNS servers supporting an UNREGISTERED INTERNAL DOMAIN. My internal DNS's recognize, but don't share and ressolve outside. I've also been running Postfix, but that isn't part of this, it's relaying. SMTP alternatives are mentioned above. My favored EMail client supports multiple persona. I point one of them supporting meORroot@mail.myinternal.domain. It seems easier.