Tracking an email

I have a web site that uses an account in my box to send emails to users of an application. He forgot his password and the application will send an email with credentials. He swears that the email is no where to be fund on his email. Is there a way to see in the mail-in-a-box servers where is the email being sent too?
Is there a log or something?

Thanks for the help,

Yes, all mail activity is logged at /var/log/mail.log

If his provider simply blackholed your email, there will be no way for you to find out.