Not in Word? (This is a Word newsgroup, in case you didn't notice.)
In Outlook, if that's your mail program, click the View menu in the message
editor and choose Bcc Field. "Bcc" stands for "blind carbon copy", which means
the addresses in that field aren't shown to the recipients.
Before you try this, check with your ISP to make sure sending to 50 addresses
won't trigger their outgoing spam filter.