MAILTO was set blank deliberately so that it doesn't spam you with emails every 20 minutes.
There for we now redirect the output to log file like this
#7,27,47 * * * * root [ -x /sbin/burp ] && /sbin/burp -a t
>/var/log/burp-client 2>&1
And remove the mailto.
And add a logrotate for that file:
/var/log/burp-client {
daily
missingok
rotate 7
compress
notifempty
}

