http://www.exchangeinbox.com/article.aspx?i=127
New-ExchangeCertificate -GenerateRequest -Path c:\install\mail_cert_request.csr -SubjectName "c=BE, o=******, ou=IT, cn=mail.*****.com" -DomainName: mail.******.com, autodiscover.*****.com, MAILSRV2, MAILSRV.*****.**, mail.****.**-KeySize 1024 -PrivateKeyExportable: $true
certreq.exe -submit -attrib "CertificateTemplate:WebServer" c:\install\MAIL_cert_request.csr
Choose the right CA, choose output folder. Open inside Issued certificates in Cert. MMC. Go to details. Click Copy to -> Complete chain, save as p7b file.
Import-ExchangeCertificate -Path C:\install\mail2.*****.com.p7b
Enable-ExchangeCertificate -Thumbprint 5B485A86***********60A04 -services IIS, POP, IMAP, SMTP
Remove-ExchangeCertificate -Thumbprint oldcertificatesthumbprint