What to look for in a mail server
Security. Ensure that the chosen vendor/developer is pro-active in publishing security patches and version updates to any known exploits. Some may say that the more patches out there means the more holes in the code to start with, and therefore the weaker the product. Really this is a furphy it could mean either their users and developers are more active in finding bugs/issues or they have a larger installed user base and have more exposure and can therefore identify more potentially harmful code.
Scalability. Don't underestimate the number of users and domains you may need to support with your mail server application.
Futureproofing. History has shown that mail server applications while critical to the enterprise are perhaps not replaced/changed as much as other enterprise applications therefore it may be worthwhile picking a robust, reliable one with a proven track history that is likely to be around and supported for a while.
Features. If you or your bosses suddenly decide that they want the latest mobile device to integrate with the mail server or that tomorrow they need Instant Messaging across the company then ensure the mail server chosen has a record of supporting the majority of the latest gadgets and gizmos available. Naturally if you are likely to never need any of this fluff then go for a bare roots mail server and save the potential headaches and extra cost.
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