How do I find out if an Email Address exists in Exchange

You want to determine if an Email address has been already configured on an Exchange server.  To do this you need to use the following cmdlet:

Get-Recipient

For Example:

Get-Recipient clint.boessen@avantgardetechnologies.com.au

You cannot use the Get-Mailbox cmdlet as remember you can configure email addresses on more then just user accounts.  Email addresses can be configured on groups, contacts even public folders.

Note: If you use Get-Mailbox with the -an switch it will only search the primary SMTP addresses.