Yes, you could have a limit on sending email. Unfortunately, PHP mail() does not give us any information about what's happening.
Additionally, it is really not recommended to use internal mail for a list of your size. This could very well be the cause of the problem. With internal mail, every single messages opens up a new running "process" on the server. It's possible that there are a ton of stalled processes (trying to resolve domain names, etc.) which has put you at your process limit and so only a few emails will get through.
It is *highly* recommended that you enable SMTP to prevent unnecessary processes. With SMTP, a single connection to the server is made for all messages to a list. (Welcome, confirmation, and notification emails are still sent with internal mail, however, to prevent too many connections to SMTP).
You may want to talk to your host about what happened because if not configured correctly the server can become bogged down. Tell them you sent 10,000 messages through PHP mail() and you think it is causing problems, and maybe suggest that they "kill" any remaining processes or stalled queue entries. Mailer optimizations should probably be made. You can view these in their own Forum.
Regards