But email is not such a simple architectural component that it should be distilled to trite answers. Email is one of the most important components of your business' communications infrastructure, often surpassing telephony, and choosing the right delivery methodology for your company is critical for your long term success.
Start by considering some basic factors in email hosting. Email systems require a good deal of bandwidth, quite a significant amount of storage, high reliability, careful management and significant security consideration.
Bandwidth is the first area to consider. Every email sent and received must travel between the end user and the email server or between the email server itself and the outside world.
In small businesses nearly all email is destined to leave the company network to go to clients, customers, or vendors. In larger enterprises email use changes. As we approach the Fortune 100 email shifts from being almost exclusively a tool for communicating with people outside the organization to being a platform primarily used for internal communications.
This shift in how email itself is used is a very important factor in deciding how to deploy email services. If email is used almost exclusively internally for intra-staff communications, this lends itself very well to hosting email systems in-house to increase security and improve WAN bandwidth utilization.
The caveat here being, of course, that a highly distributed company of any size would not keep this traffic on a LAN network. And so the solution should be treated as if the email usage is external regardless of whether it is intra-staff. Small companies with communications happening primarily with external users will find better utilization in a hosted service.
Storage is actually often a smaller factor in email architecture decision-making than it may at first appear that it should be. Traditionally email's storage requirements made a compelling argument for hosting internally, due to the cost benefit of keeping large storage local (especially that used for archival needs).
Recently, large hosted email vendors such as Rackspace and Google Apps have brought the price of online, archival email storage so low that, in many cases, it may actually be more cost effective to utilize hosted storage rather than local storage. Or at least the cost is at parity. Even long-term archival storage can be had very cost effectively in a hosted solution today.
Reliability is a rather complex subject. Email is critical to any organization. If an email system goes down many companies simply grind to a halt. In some cases, the company effectively shuts down when email stops flowing.
Hosted email has the obvious advantage of being hosted in a large, commercial datacenter with redundancy at every level (assuming a top tier vendor) from hardware to storage to networking to power to support.
In contrast, hosting email in-house requires a business to determine the level of redundancy that is most cost effective given the business' ability to withstand email downtime. This is generally an exercise in compromises - how much reliability can a company do without given the cost necessary to provide it.
Some companies will opt to host email servers at a colocation facility, which will provide them with many redundant components. But to meet the features of a Rackspace or Google level offering, multiple datacenters would likely be needed. Colocation is a halfway option providing the technical features of hosted options with the management and flexibility of in-house email systems.
A more common scenario, though, is for companies to host a single email server completely within their walls, relying on their internal power, hardware and network connection. In this scenario a company must either take extreme measures to ensure uptime - such as hosting a completely redundant site at immense cost - or front-end their entire email infrastructure with a reliable online spooling service such as Postini, MessageLabs or MXLogic.
The cost of such services, while critical for the reliability most companies need, is often equal to or even greater than complete email hosting options. This spooling service cost will likely add an ongoing, scaling cost that will make fully hosted email services always a less expensive option than in-house hosting.
Management cost is very difficult to determine but requires attention. A fully hosted solution requires relatively little technical knowledge. Time to manage is low and the skill level necessary to do so is relatively low.