-Could we utilize the following big providers: outlook, Gmail, yahoo, etc. with our own domain? #Yes, it could be done with google (google apps) and outlook. -If not what is the estimated cost of ready made software or custom built email? # Well there are many reliable and easy email provider from which you can build your own email startup. Explicitly spending for hardware and the software will be very costly. These include ZohoMail, FastMail, Pobox, RackSpace, SocketMail. I would recommend the latter two. Conclusion: I would recommend you to go for google apps (gmail) for your startup, it will cut your cost security wise which is a major investment now a day. As security today is a number one priority. Google apps would also be a hassle free setup, only updating few things. Major down side it the gmail layout which will remain there but if it don't matter your all good.