Let us manage your UNIX-based….
Did you know that you can have your own server for as little as a few US$ per month?
Virtualization technology now allows anyone to rent a slice of a physical server at a hosting company. This "slice" is called a "Virtual Private Server" (VPS), and looks and behaves almost exactly like a physical server. Which is to say, the renter has complete control over what software is installed on the VPS, and how it is configured. A VPS is also typically highly configurable in terms of how much memory, bandwdith, and hard disk space is assigned to it, and these details are what determines the cost of rental.
A simple e-mail server (in fact, most simple, stand-alone servers that are not heavily used…) would easily fit into the cheap few US$ per month end of the spectrum of VPS offerings. And if a VPS turns out to not have enough horsepower to do what you are asking of it, adding resources (which adds to the monthly cost) is trivial.
Typical reasons for renting your own server might be:
And of course, having your server hosted by a commercial hosting company is the ideal scenario for also out-sourcing management of the server to a cheaper jurisdiction. Hosted servers are by definition setup for ease of remote access and administration. Not to mention, the hosting company provides s static IP address and 24-hour monitoring and support of the server hardware, services that are seriously non-trivial to provide at the location of your small business.