Maintaining Exchange SMTP Addresses with Recipient Policies

Most of the time when I'm setting up a network, or upgrading a server, the Exchange email addresses differ from the Windows user name. Like the usernames might be FirstnameLastinitial, while the email addresses are FirstinitialLastname@example.com. You can use recipient policies in Exchange to automatically maintain the correct email addresses for all your users.

You can add a new recipient policy, or edit the default. For example, to use FirstinitialLastname@example.com, you can edit your default recipient policy to change the default "@example.com" to "%1g%s@example.com" (the first letter of the given name, and the entire surname).

How to Modify an SMTP E-Mail Address by Using Recipient Policies - Microsoft KB article