did you try the follow-up steps
Enable the Certificate
Enable-ExchangeCertificate
-Services "SMTP,POP,IMAP,IIS"
Enter the Thumbprint of your certificate (i.e. 9292D650DFFD7E055145E5CA5A
29E08DFC07
C53C)
Select Yes To Overwrite
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then configure exchange to use SSL
http://technet.microsoft.com/en-us/library/bb310764(EXCHG.80).aspx