Chances are it's your Internet Service Provider (ISP) that's preventing your message from being sent. Your ISP might do this, for example, to deter folks who send unsolicited bulk commercial email (such as "spam" or "Junk mail") to a large number of addresses.
If this happens to you, contact your ISP and ask them if they enforce a limit on how many recipients each email message can have. If there is a limit, you can either send your message to fewer people, or divide your recipient list into multiple sets and send the same message in batches (each time to a different set of recipients).