Select Menu > Messaging and Options > Settings > E-mail.

From here you can...

Add an inbox by selecting Mailboxes > Options > New mailbox. The name you give to the new mailbox replaces Mailbox in the Messaging main view. You can have up to six mailboxes.

To remove a mailbox and its messages from your device, select Mailboxes, tap the mailbox to be removed if it is not already highlighted, and select Options > Delete.