Best if you do it from domain registrar panel
hi,
if your domain registrar has facility like managed DNS records, you may set it with proper mx records with priority and mails should be sent to a different server when they are sent.
Many people now use google mail solution to use their domain mail on google a/c's they do it this way, i think setting mx records on server where site is hosted would certainly bounce those mails in case the box is not responding and mx records would not be re-directed.
see ya,
mohit