Exchange 2007 creates a self-signed certificate during installation that uses all the server and domain names that are known to Exchange at the time of installation. These certificates are valid for 12 months. In some cases, it may make sense to clone these certificates if the Subject and Subject Alternative Names can be used for other computers. Be aware that only the certificate metadata and not the key sets are cloned.
To run the following cmdlets on a computer that has the Edge Transport server role installed, you must log on by using an account that is a member of the local Administrators group on that computer.
To clone a new certificate from an existing certificate, you must first identify the current certificate for the domain by running the following command:
Get-ExchangeCertificate –DomainName mail1.mydomain.com
The first certificate that is listed in the output is the default SMTP TLS certificate for the server. To clone the certificate, run the following command:
Get-ExchangeCertificate –Thumbprint c4248cd7065c87cb942d60f7293feb7d533a4afc | New-ExchangeCertificate
Where the value for Thumbprint is from the first certificate that was listed in the output for Get-ExchangeCertificate.
This command extracts the names from the existing certificate that are identified by the thumbprint and uses them in the new self-signed certificate.
Source: Exchange 2007