how to check if an email was ever sent

You can look in the logs if you have logging turned on.
Locate your log directory
1) System Manager - First Admin Group - Protocols
2) Right click on SMTP and select properties
3) Near the bottom it should have a check box in Enable Logging
4) If it does you click the Properties box and it tells you were the log folder is located: Default is
C:\WINDOWS\System32\LogFiles
5) Once there your looking for a folder with a name like SMTPSVC1 (if you ahve multiple SMTP services there will be multiple folders).
6) Inside that folder are the logs. Easiest thing to right the SMTP folder and select Search. Where it says File Contains you type the email address. Windows will search through all the logs for that particular address and return which files have it.

If you don't have logging turned on then I don't believe there is a way to find out.

Or use Message Tracking under Excange System Manager - Tools.

Unfortunately the default log retention time is only 30 days if memory serves so might not be conclusive.

You don't have any form of archiving / journaling like Enterprise Vault?

Neill