Mail users have spam sent to their secondary SMTP addresses forwarded to their personal e-mail addresses- Exchange 2010

Question

We currently have a spam issue relating to the secondary SMTP e-mail addresses of all our Mail Users.
Example: Peter Smith has an AD account on our company.com domain and an associated external e-mail address ([email protected]) i.e. he has an AD account ([email protected]) with Mail User Exchange attributes so that he can logon
to our SharePoint server. However, this AD account also becomes a fully functional secondary SMTP address and all spam sent to [email protected] also gets forwarded to [email protected]
To mitigate this issue, I have added this secondary e-mail address to the "Recipient filtering properties > Block messages sent to the following recipients" in Hub Transport/Anti Spam.
Two questions:
1. Is there an easy way to delete the secondary SMTP addresses of mail users? I was unable to (it just reappeared shortly after deletion)
2. What is the PS command to export a list of all secondary SMTP e-mail addresses OF MAIL USERS and then adding this list to "Recipient filtering properties > Block messages sent to the following recipients". I could manually add them
in EMC but there are quite a few of them...
Many thanks for your help!

Answer 1

Now that I understand what's happening, let me see if we can get a more accurate solution for you.  So your mail users (Exchange calls them mail-enabled users, as opposed to mailbox enabled users, who have mailboxes) have SharePoint access (with the
@company.com address), but have actual mailboxes externally (the actual end-domain is inconsequential).  You want users of these systems to stop receiving emails on the company.com namespace, but to receive them from only your SharePoint system. 
A "textual diagram" is below:
SharePoint (to Exchange mailbox owner) ==> Exchange hub (using company.com) ==> Exchange mailbox
SharePoint (to External mail user) ==> Exchange hub (using company.com) ==> Exchange mail contact ==> External system (using external SMTP address
External Messaging system (to External mail user, using company.com address used by SharePoint) ==> Exchange hub =XX=> Block delivery
You wish to stop the third of these from happening.  If your SharePoint system used non-routable addresses (such as company.local) and all of your mail users had only these addresses added to their accounts for internal mail precossing (as the
internal name used by Exchange), this would solve your issue.  The question becomes how to deploy this sort of address. 
If these are the ONLY mail contacts in your organization, the solution is simple - you almost had it above, but may have missed seeing it.  Policies are applied on a priority basis, with the default policy applying only if no others apply.  Since
these accounts are considered "Contacts with external e-mail addresses", you can create a policy for your entire organization that handles only these accounts.  You don't need to apply it to a single OU, if your mail enabled accounts aren't
in a single OU.  (If they are, of course, feel free to apply it only there.)  You can create the policy and apply it to the organization, but only to those objects that are "Contacts with external e-mail addresses".  That way, it won't
affect any of your Exchange mailboxes (current or future).
Does this help?
(BTW, one minor correction - your third statement about mail contacts is incorrect - Exchange sees no major difference between mail contacts and mail enabled accounts - and in my experience they are both addressable by the internal address.)