From the man page for at/batch, it says
The user will be mailed standard error and standard output from his commands, if any. Mail will be sent using the command /usr/sbin/sendmail. If at is executed from a su(1) shell, the owner of the login shell will receive the mail.
But how do configure what email address its sent to?
In cron can use the MAILTO enviroment variable, but can't see that anywhere within