Questions:

What are the maximum number of recipients I can bcc on an email message in Outlook?
What happens if I exceed that limit?
Do I get any kind of error or warning message, or do some people just not get the message?

Answer:

The maximum number of recipients in an email is 100 - that includes To, CC, and BCC all together. That's a very common limit, not just at MIT - it's generally good to stick to the conventional limit, because it would be bad to send a message which MIT accepted but then the final recipient ISP rejected because it had too many recipients.

We have confirmed that we get a bounce message from the mail servers when we try to send to >100 recipients (using Outlook 2010). This is sent by the servers, not by the mail program itself, so you should get it no matter what mail program you use.