Microsoft limits the number of people you can simultaneously send a single email to in order to deter spammers. That number is based on your account status, including the age of the account and whether your account is free (anonymous) or paid (verified). There is no way around this restriction, so you'll need to break the email into groups to send it to all desired recipients.
P.S. This is a restriction placed on Windows Live Mail (Hotmail), the email service, not the desktop email client that can be used with any IMAP/POP3 email service.
Windows Live Mail refuses to send mail to my contact groups (16 contacts) with following error: Unable to send or receive messages for the Hotmail account. The message you're trying to send has too many recipients.