Exchange 2003 and Single Instance Storage Question

After searching MS and EE I still have questions regarding SIS.   My understanding of this concept is as follows.  

Copied from EE:

"Single Instance Storage is per Database, not Storage Group. The Storage Groups structure is designed primarily for sharing a set of transaction logs for all the databases. Remember SG's can have more than database or store which share the same set of transaction Logs.  If you ever have to go through a disaster recovery with a tape restore, this new architecture design becomes crystal clear. ;-)  Also, SIS is per message, not just the attachment. This means its the body + attachments.

Anyway, here's the scenario:
A user sends a 20 mb file attachment to 10 different people. These 10 people are 2 per store, 5 stores, 2 in SG1, 3 in SG2.  The distribution would be message body+20 Mb attachment per store.
SG1\mbstore1 gets body + 20 mb accessed by 2 people
SG1\mbstore2 gets body + 20 Mb accessed by 2 people
SG2\mbstore3 gets body + 20 mb accessed by 2 people
SG2\mbstore4 gets body + 20 mb accessed by 2 people
SG2\mbstore5 gest body + 20 mb accessed by 2 people

A reply to the original probably be negligible in size for 2 reasons:
1.  A reply would go back only to the originator (1 person) with a little addition to the body.
2.  Replies do not by default include the original attachment. Only Forwards do."

My question is does SIS still apply if the sender is outside of the exchange environment?  So if a sender is a google address and they send a 20mb file to 10 users within my Exchange 2003 environment, does the message once received by my exchange server SG1\mbstore1 consume 20mb or 200mb?


cspantherAsked:
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AkhaterConnect With a Mentor Commented:
SIS will apply even to emails sent from outside the org, in your example, if the 10 recipients are in the same mailbox database it will consume only 20 mgs and not 200
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cspantherAuthor Commented:
Thank you for the assistance.
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