Any limits tot he number of emails will be imposed by the server.
you should check with the server to ask about limits on sending emails using PHP
It is certainly posible that it could be a bad email address in your mailing list. does it always stop at the same point in the mailing?
To be able to troubleshoot, I would need a test mailing set up that demonstrates the problem, I need to be able to reproduce the problem in order to troubleshoot the cause