I didn't think you could use HTC app's on a ROM that wasn't stock but if you can that's even better yet.
Easiest way to get a hold of that is to flash back to factory and pull it from your phone. Go to the new ROM you want and push the apk back over.
Here ya go I put it together for ya. This has the htc mail apk and the widget. Just put your phone into recovery mount the system and push to the app folder. And you off!
http://www.pyxistech.net/htcInc/HTC_Inc_mail.zip
First place those files onto your sd card's root.
Once you get into recovery go into partitions. Then mount the /system partition.
Open your command prompt.
adb push 'location of file to push/name of file to push' /system/app
Do that for each file. Reboot your phone. So if you stored your files on the sdcard it would look like this:
adb push /sdcard/htcmailwidget.apk /system/app
Remember they have to be capitalized correctly! Hope this helps you get started.