You may wish to limit how much a user can store on a server, so you are not in constant danger of running out of server disk space. Some e-mail software allows you to set up controls on eligible recipients and the size of messages they can send. This may seem far fetched, but we have seen it attempted before. Twenty to 30 MB per user is a good measure to size your servers. This will make it easier for you to support and possibly allow you to get volume purchase discounts.
Standardized hardware allows the efficient cloning of properly configured mail servers.
It is extremely important that you have a reliable backup and restore system in place. Depending on the e-mail software, you may need a spare server in order to restore a post office or public folder. If the e-mail system you choose does not rely on the underlying operating system for security, you will have to develop standards to take this into account. You will want to follow any security standards already in place at your company, such as password length and expiration. If your company is international, there will be implications on what types of encryption that can be used.