Maximum number of voters in Outlook 2010

Hi,

When using the voting option on Outlook 2010 is there a maximum number of voters who can participate?  We are using Outlook 2010/Exchange 2010 and recently an email with voting was sent out to a large number of recipients (700+) and it seems that only half actually had the voting option (the actual numbers are unsubstantiated everyone just keeps saying half).  Looking for a hard metric here if there is one.

Thanks.