Exchange mailboxes can have multiple attributes set against them...Department, State, Address, Phone Number, etc, but what if you need more than that?  Typical questions I have been asked are:

• How do I tag a mailbox as an Alumni
• How do I tag a mailbox as a male/female user
• How do I tag a mailbox as ...?

Using PowerShell, if you run the Get-Mailbox command against a mailbox (e.g. 'Get-Mailbox Jonny') you will produce a rather large list of complete set of attributes, and right in the middle of the list, you will see CustomAttribute1 thru CustomAttribute15, which if you have not used them until now, will be blank.  These attributes can be used to store more or less any data you want.  You can then tie things like Dynamic Distribution Groups, applications, or something else to them.  The only thing an admin cannot do is rename the attribute itself, nor can you create new attributes, so you will need to keep a note of what each custom attribute is for.

These attributes can be updated quite easily...here is an example of how:

Lets say you wanted to use CustomAttribute1 to tag Jonny's mailbox as an Alumni:

Set-Mailbox jonny -CustomAttribute1 'Alumni'

That's it.  You can also make reference to these attributes when doing bulk mailbox creation.

Jonny