Kinda odd that it reverts. Should be able to drill into the messaging options and select a notifier and then you can use the default notification in the main settings for emails.
I haven't done it that way in a long time however. I use SMS Popup which has independent notification settings. Then you can set the default notification to whatever you want for emails or if you use a non HTC email program like K9 mail, that app will most likely have it's own notification settings. Similarly, I use Google Voice for voicemail and once again, it has it's setting for voicemail notifications.
So you SHOULD be able to do it with the onboard HTC settings but many people end up using 3rd party apps that take over the job of notifying. That said, often times when you enable notifications on a 3rd party app, you need to DISABLE the default notifications so you don't end up getting multiple notifications for the same event.