If offline e-mails are available then you will be able to access your mailbox without having a connection to the e-mail server. This is possible if mailbox content has been copied to local folders on the e-mail server.
As soon as a copy of the e-mail exists in the local folder, the e-mail will be available offline. Even if you do not have direct access to your mailbox on the e-mail server, you will still be able to edit, write and delete new e-mails and so on. Sending, and deleting new e-mails is only possible when you switch to online modus.
E-mails available offline are not CAS genesisWorld data records.