It does depend a bit on your software level. Here is MCD 5 setting from the help files:
Programming
To enable the Forward to Email feature for the system, perform the following steps for each node in the network/cluster:
1.System Options form.
Enter the IP address of the default Email server in the "Email server" field.
2.VM Network Server (For single-system configurations only; not required for networked voice mail.)
Enter the DNS name of the controller.
3.For each user, enable Forward to Email using the VM Mailboxes or User and Device Configuration form.
4.Make sure that your local Email server is set to accept Email from this node, and that attachments are allowed. See your Email server documentation for more information.
##Notes:
##When "Delete After Forwarding" is enabled, messages are only deleted after successful delivery to the destination Email server. The Message Waiting Indicator light turns on when the voice message is received in the voice mailbox, and turns off when the message is deleted from the voice mailbox.
##When "Delete after Forwarding" is disabled, your telephone's message waiting indicator continues to flash even after the message is forwarded to Email. To ensure that your mailbox does not fill up (which prevents new messages from being left), periodically access your mailbox and discard unneeded messages.