It should be pretty simple - If Get-Mailbox shows the emailaddresses property for a mailbox, and Set-Mailbox allows you to use the -EmailAddresses switch to add email addresses, one can't be blamed for believing it'll work the same way for Public Folders.
Objects other than Public Folders need to be mailbox or mail-enabled to be Exchange recipients, Public Folders do not (Yes, they are mail-enabled by default). To modify mail-related attributes of Public Folders, you use the Set-MailPublicFolder command.
To add additional email address to a (mail-enabled) Public Folder:
$PF = Get-MailPublicFolder "Sales"
$PF.EmailAddresses += "Sales-EMEA@domain.com"
$PF | Set-MailPublicFolder
The first line gets mail-related properties of Public Folder "Sales" in a variable called $PF. Next, we add the additional email address, without wiping out the existing ones. Finally, we commit the change using Set-MailPublicFolder.
If you simply use Set-MailPublicFolder "Sales" -EmailAddresses "Sales-EMEA@domain.com", it will replace the existing values in the EmailAddresses property.
Another difference to note between how the Set-PublicFolder and Get-PublicFolder commands work, compared to Set-MailPublicFolder and Get-MailPublicFolder - the former takes a relative path of a Public Folder. For instance, to get the Sales PF if it's in the root of the Public Folder tree, we would need to add a \ before the name:
Get-PublicFolder \SalesHowever, the Get/Set-MailPublicFolder commands work using the alias/display name of the PF. Why the difference? One way to look at it - when using Get/Set-PublicFolder, you're working with the actual Public Folder. When using Get/Set-MailPublicFolder, you're working with the Active Directory object created for that Public Folder (which holds mail-related attributes, making it possible for a Public Folder to be mail-enabled).
To change the primary email address of the Public Folder "Sales" from "Sales@domain.com" to the new address we just entered - "Sales-EMEA@domain.com":
Set-MailPublicFolder "Sales" -EmailAddressPolicyEnabled $false -PrimarySmtpAddress "Sales-EMEA@domain.com"As you may have already figured out, we exempted the Public Folder from getting EmailAddressPolicies applied. In Exchange Server 2003/2000, you could change the default email address of a recipient, without unchecking the checkbox. Result: A few minutes after you completed the change, Recipient Policies would apply and change the primary email address back.
Exchange Server 2007 doesn't let you change the default email address without exempting the recipient from email address policies.