Also, bear in mind that after the LUN is shrunk, you should also resize the volume containing this LUN - unless it's thinly provisioned.
On that note:
Instead of resizing anything you may consider changing volume guarantee to none & disabling LUN space reservation - it will take from aggregate only space required for data (plus any potential 'holes' after files being deleted), regardless of the LUN / volume size.
If you are using a dedicated SnapInfo LUN for your Exchange environment and you can't shrink the LUN any further, just create a new SnapInfo LUN that is smaller and then re-run the SME configuration wizard to move the SnapInfo data to the new SnapInfo LUN.
You can move the SnapInfo data without disrupting Exchange services. Just be sure to not move any databases or transaction logs.