This is how you investigate email generation
1) - Turn on mail.debug in Administration | Support | Email -> "Enable Debug Mode?" = True.This will start to log every outgoing email
The caveat is that you will have to know at what time the alert came throug that you are tracking down.
You can get this from the alarms dashboard for the Alert the customer said failed to be emailed.
2) Trigger the alarm in question and review the latest FMS log and you will see the following messages (per each email you will see the following)
=> STDOUT - DEBUG SMTP
This is the start of the mail debug
=> STDOUT - RCPT TO
This is the Mail to person
=> Subject:
The subject of the alarm that was sent
=> Queued mail for delivery
Email sent to SMTP
=> STDOUT - 221 2.0.0 Service closing transmission channel
SMTP connection closed.