Beyond the basics of providing messaging services, the issue is how these services are provided. Can the product scale? Is the product secure? How hard is the product to install and manage? How easily can users be provisioned? How flexible is the product? There are many messaging products out there, and each of them is architected and designed slightly differently. One product may store user names and passwords in a flat file, while others leverage LDAP. One product may provide integrated antivirus measures but not allow you to integrate a slightly better third-party product for antivirus protection.
There are several key items:
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