As far as I could find out from the Microsoft forums its 255. Here is a brief bit of info.
I was designed this way to avoid potential DOS attack.
It supports a maximum of 365 Accounts.
Many isp’s will not allow that many recipients on a message. If you use anything but exchange server, there is usually a liimit based on the length of the address string. If you are using an Exchange server, the limit is whatever the admin set the max allowed recipients.