Yes, you can use e-mail certificates on several computers. To do this, you must first export the certificate with the private key.
This is how you proceed if your e-mail certificate has been saved in Outlook:
Outlook => File => Options => Trust Center => Settings for the Trust Center =>
Email Security => Import/Export => Export Digital ID to File
If the certificate with the private key was successfully exported, you can import it to another computer, Outlook, or another e-mail client.