How to Import and Enable a Certificate for use with Exchange 2007

Copy the SSL certificate from the email from you provider and save this as mydomain.cer. Remember the name and location.

Open the Exchange Management Shell and run the import and enable commands together separated by a pipe character:

Import-ExchangeCertificate -Path C:\mydomain.cer | Enable-ExchangeCertificate -Services "SMTP, IMAP, POP, IIS"

If you have an exported certificate, it must be exported with private keys. to import an exported certificate:

Import-ExchangeCertificate -Path C:\mydomain.pfx -Password:(Get-Credential).password | Enable-ExchangeCertificate -Services "SMTP, IMAP, POP, IIS"

a credentials box will pop up on the screen prompting for username and password. It doesn't matter what you put in the username field, but you have to put something. Then enter the password that was set for the certificate when it was exported in the password field.

References:
http://technet.microsoft.com/en-us/library/bb124424.aspx

http://technet.microsoft.com/en-us/library/aa997231.aspx