Azure Web Roles and Instances

I got a situation here and i'd like to know if someone had the same situation and knows what to do.

I got a web role with multiple instances. I'd like to send every morning at 7AM an email to my customers with some information about their statistics. Now is the problem If I make a Cron job for it the mail will be send more then 1 time.

How can I solve this problem that only one of the instances will send this email?

Already a big thanks!