Martin, When you have exchange 2007 the certificate should not be enabled using IIS console. You have to use the powershell to assign the certificate. User might have been getting the error message because of the name on the certificate. You can use the SAN certificate to be used with your exchange server. Please see the links below which should be good enough to walk you through..
http://technet.microsoft.com/en-us/library/aa998327.aspxhttp://technet.microsoft.com/en-us/library/bb124424.aspxhttp://technet.microsoft.com/en-us/library/aa997231.aspxhttp://www.digicert.com/subject-alternative-name.htmM