Have you accidentally activated the Mute function - either by the switch by the side of the volume control or by the soft switch in the multi-task bar? Either location can be programmed to be the Mute function, it's settable in the 'Settings' menu and it mutes 'system' sounds, such as the email sound.
Thank you so much Tim. I changed the setting of "lock rotation" to "mute" function, unmuted it with the small button next to volume control and the outgoing e-mail sound now works.