Not really much point. Email as a protocol isn't secure, so they can always use some software to read the messages in transit even if they secure them at rest. Many systems use Dovecot, Postfix, and other systems that have been open for ages. Only the UI is likely proprietary and that's mostly just for viewing and billing. And anything they're doing that's nefarious is likely a separate code base entirely since it doesn't need integration with the data in motion not being secure.

What are you looking to gain by understanding their code base?